Dome Light Issues after Auto-Dimming Mirror Install

Hi folks.

Over the weekend, while my car was disassembled, I installed a Fiesta Titanium Auto-Dimming Mirror. I followed the procedure roughly outlined in this thread here: https://www.fiestastforum.com/threads/fiesta-titanium-auto-dimming-mirror.6840/

I tapped into the wires in the map light console for power and ground, as indicated, and then ran a wire down the a-pillar and tapped into the +12v reverse wire in the kick panel.

However, this has caused an admittedly hilarious issue. When the mirror is plugged in, the rear map lights will not turn off. Not the dome light, just the two individual aimable map lights. Additionally, when the front map light switch is set to "door," the front map lights only dim about halfway instead of turning off. Flipping the switch to "off" turns them off all the way. Unplugging the wiring harness from the back of the mirror immediately resolves the issue.

Unfortunately, D1JL has passed and Canadianguy hasn't been on the forums since 2019, so I can't ask either of them for their help.

Can anyone think of what went wrong here? I am trying to avoid adding more add-a-fuses and other such things into my fuse box - and god, I do not want to have to pull the headliner and A-pillar trim again - but I am curious generally as to what's going on and how to fix it.

Cheers!

Do you have LED or incandescent bulbs installed?

Is there an IC built into the rear light assembly for dimming? If so, ICs can be sensitive to an insufficient path to ground.

I had some aftermarket headlight LED bulbs, engineered with load resisters in them to trick vehicles into thinking they had normal bulbs.
After some years, that circuit began to malfunction, dropping load off to ground in "rapid" pulses.
This messed with an IC in the BCM that was responsible for regulating intermittent wiper function.
Replacing the faulty LED bulbs resolved the issue.

With everything connected and running, (rear defrost, blower on high, headlights, brake lights, stereo, engine at 3,000 RPM, etc...) voltage drop test your ground circuit, from the LED dome light, back to the battery negative. A loss of >5% is considered excessive.

I am of course, assuming that the mirror wiring is verified, properly connected. If the wires are mixed up, that could be responsible for dumping something off to ground.

Interior bulbs are all LEDs (my anti-LED status applies only to headlight bulbs ). I am not sure if there's an IC, but I don't believe there is, they're just basic COBs. I don't have reason to believe that the wiring is mixed up, I got the pins and the wire colors correct.

I'll try switching 'em out for some regular halogens, and if that doesn't solve the issue I'll do a voltage drop test. How would I do that? I have a multimeter, I just haven't done this before.