Just like my car's head unit, my car's rearview mirror works fine. However, again taking inspiration from my dad's 2019 Acura RDX, I wished that my mirror could be "frameless" (as seen above) rather than have the thick bezels which surrounded its glass.

Ordinarily, replacing a rearview mirror is pretty simple. You might have to unscrew a screw, but you simply slide the mirror off/on:

Another factor slightly complicates things – many (if not most) rearview mirrors nowadays are auto-dimming:

However, this isn't too much of an issue, because these mirrors were designed really well. They only need power (+12V and ground), and their internal circuit board takes care of the rest - deciding when to dim.

Okay, so simple enough. I can just slide my old rearview mirror off, put the new one on, and connect it to the old mirror's power and ground wires, right? Well, there's one more thing...

Here's a picture of what my car's mirror looks like from the side. You may have noticed that there's a camera attached to it, which is there for my car's automatic high-beam system ("high-beam support system"). In case you're interested, it's a technology largely developed by Gentex Corporation, a company which produces the auto-dimming rearview mirrors for a significant portion of cars sold nowadays.

It's not a terribly sophisticated automatic high-beam system: it's simply on/off, whereas some systems dynamically block only the portion of the high-beam which projects at other cars with some sort of matrix system. Regardless, I wanted to keep the feature if possible when installing the new rearview mirror.

My initial plan, or hope, was that I could open up the donor "old" mirror (the one that was identical to my original mirror) and find two printed circuit boards (PBCs): one for the dimming of the mirror glass, and the other for the automatic high-beam system. I was hoping that most Honda Accords had identical rearview mirrors, but this one simply had another PCB stuffed inside of it to connect to the camera.

So, I did a teardown of the donor "old" mirror to see what was inside. The full write-up is here, but here are the main takeaways for the purposes of this project:

There was only one combined PCB, though I had hoped for there to be two: a bigger one for the mirror, like all auto-dimming mirrors have, and a smaller one for the high-beam support system.

This letdown actually turned out to be a blessing in disguise. See, my original plan was to take that small auxiliary PCB and somehow hope that I could stuff it into the new rearview mirror's enclosure, a plan which rided on a lot of things fitting together perfectly.

However, now that doing so was impossible – as this PCB was physically larger than the new mirror – I was forced to find a different solution. While FaceTiming my friend, I realized that this project didn't have to be so hard at all: I could just 3D-print a case for this PCB and stuff it in the dash somewhere. As long as I had enough wire, I could split and extend 1) the factory's roof harness connection, and 2) the camera's connection over to wherever I stashed this PCB.

This part of the teardown also worked out in my favor. I say "worked out", but really, it "worked out" because the engineers behind things like this do a great job, designing things to be simple and modular.

The fact that the camera detached from the mirror so cleanly, with this metal attachment insert and screw, meant that I could make my own adapter, inside of which I'd put the metal attachment insert, then screw it into the camera mount. That adapter could then be designed however was necessary (as I planned to 3D-print the parts) to screw the new mirror into it.

Thus, this system provided me with an extremely simple, elegant, and robust method of securing my new mirror to the existing camera/mount. Another benefit was that the high-beam support system camera was exactly in the same place as before, as I was using its built-in mount rather than having to fashion my own method of attaching it to the new mirror.

I should probably note, in case it's not obvious, that changing a rearview mirror is indeed easy, but the mounts sometimes differ between cars: some cars are slide-on, slide-off; some are twist-on, twist-off, etc.

Since the plan was to stash the PCB in the dash somewhere, I had to 3D-print a case for it. Now, this was a turning point for me. Up until now, whenever I wanted to 3D model something, I would use a ruler and make measurements, which was very tedious, but very precise, symmetrical work. With this, however, I just took a top-down picture of the PCB, imported it into Autodesk Fusion 360, and sketched an outline around it. This was so much faster and easier than taking all of those measurements, but the downside is in precision.

Since it's an imperfect picture that I'm using as a reference, if I were to actually follow it, I would get many lines that are at 177°, 92°, etc.: they would be a little bit off. So, the strategy here was to use the image as a reference, but pretty much guesstimate what the actual dimensions were supposed to be. An additional issue was perfectly sizing the image to calibrate the size of the model, which was stressful but seemed to work out all right.

Anyway, here's what I ended up with:

Well, that was simple enough. Fortunately, the PCB fit the first time, so there wasn't a need to reprint. Also, this was going to be stuffed into my dash somewhere, so it didn't have to be especially strong. So, I went onward to the next phase of the project: wiring.

Parts

Project-Specific

Item	Qty @ Cost	Notes	Link(s)
Donor "Old" Mirror	1 @ ≈ $40	Honda Part #76400-T2F-C01 I bought mine from LKQ (a national salvage yard chain) on eBay, but it was the only one on eBay that I found. I'd suggest using car-part.com and look for a 2016/2017 Honda Accord rear-view mirror with "automatic dimming" and "rain sensor." In this case, it is not a rain sensor – it is the automatic high-beam system camera – but it has been entered into the database as such.	car-part.com
Donor "New" Mirror	1 @ $45.00	2019 Acura TLX auto-dimming rearview mirror I bought this from eBay as well, but this could theoretically be nearly any auto-dimming rearview mirror, so long as you devise a way to secure it to the old mirror's mount.	eBay
Inline 3-pin Male & Female LED Connectors (20 pairs/pack)	1 @ $10.99	Previously, the rearview mirror plugged directly into the car's roof harness. However, I now had a custom "splitter" harness between the mirror and car, so I bought these to connect the mirror to it. That way, if anything went wrong with the mounting system I created*, I could easily remove the mirror without cable-managing again. Red: +12V (IGN1) *One of the only times I can recall that I've successfully thought ahead. Yes, that's foreshadowing...	Amazon
Automotive harness cloth tape	1 @ $10.99	Used to "black out" wiring and hopefully give everything a near-factory look.	Amazon
15 ft. 24/28 AWG Micro USB cable	1 @ $5.94	Used as a 5-connector wire, to split the car's 4 connections among the new and old mirror as detailed below. Tucked in roofliner around the windshield like a dash cam.	Amazon
25 ft. Cat6 Ultra-thin Ethernet Cable	1 @ $8.49	Used as an 8-connector wire to connect the camera (which used exactly 8 wires) to the old mirror's PCB (hidden in the dash). Tucked in roofliner around the windshield like a dash cam.	Amazon
Grand Total

Now, unlike my first car project (this one being my second), I didn't spend weeks planning in advance for this. After all, the first project entailed relatively much more wiring difficulty, requiring that I make my own circuits with relays in order to switch on the radio. So, feeling a bit more confident due to the simplicity of this project as well as my slight experience, I didn't draw up a schematic ahead of time; the parts arrived, and my impatient self got right to soldering.

Below is what the custom harness looked like, which plugged into the factory's roof harness and:

1. Split power/ground to both the new mirror and stashed PCB
2. Retained CANBUS connections to the old the PCB, so that it could read the camera as it did before, but send its output – its decisions as to whether the high beams should be on/off – back to the car via the roof harness
3. Allowed me to connect to the BACK_LT connection of the new mirror from wherever the PCB was stashed – rather than running a separate wire to the mirror, which would be messier. Pretty much an extension wire.

With the wiring complete, I then turned my attention to wondering how, exactly, I was going to adapt the camera arm's mounting system to the new mirror. I eventually ended up taking measurements (as images were not as helpful in this case) and coming up with the following pieces:

These pieces secured together as follows to attach the new mirror to the old mount. The left and right brackets were attached (via the silver screws) to the new mirror, then those brackets were screwed (via the smaller black screws) into the core piece, which contained the metal insert from the old mirror that secure it to the mount.

It seemed like a rather robust design. So why was there a need for a second attempt? Well, one day, while I was busy for a couple of hours, my car was outside in the beating sun. I came back to this:

So, I learned something that, evidently, every 3D-printing novice already knows: PLA, a very common plastic for 3D-printing, is not very resistant to heat.

I began looking for some other materials that I could use, and I settled on a carbon fiber-infused nylon compound. Now, I got such a good price on the print, that I'm not really sure that it is indeed carbon fiber infused, by even nylon 12, by itself, is strong enough. I got a black finish, and the seller sent me the following picture:

We'll see how it goes! Fingers are crossed for now.

Thank you very, very much to the following people for their help and support on this project:

• My family
  
  Again, thank you so much for letting me mess with the wiring of the car. Although electrically simpler, this project was a bit higher risk than the first one because I wasn't building an isolated circuit; I was tapping directly into the factory wiring for the mirror. Thank you especially for putting up with my silliness, as this project was just about as close to unnecessary as it gets.


• Professor Sims
  
  Thank you very much for your feedback on my website and for all of your help. If it weren't for you, I don't think I'd have ever put in the time to complete this write-up (among others). Thanks to you, I will always have these project stories to look back on.