There are 2 companies that make auto dimming mirrors. Donnelly, which alot of GM cars have from the factory, and Gentex, which is the preferred mirror to have installed. The Donnelly have a history of leaking fluid from them when they age, if this happens your auto-dim capabilities will not work anymore. The gentex mirrors from 98-02 Olds intrigues have LED map/dome lights that are electronically controlled. Instead of hard switches for the map lights that lock in an on/off position, the map light switches are rubber soft-touch switches, same as the autodim on/off. This seems pretty cool at first, but that also means the electronics in the mirror stays powered up with the hot feed so that the map lights can work. Worse, for some odd reason I can't figure out, the VFD filament is hard-wired to the hot wire as well. This means the hot/standby circuit in this mirror uses 200mA at all times (checked with my multimeter). That much load will drain a good battery in just a week or two. I had opened my mirror and wired in a tiny relay to kill the filament when the ignition is off which worked fine, but the electronics in the mirror (microcontroller) for the domelights and such still uses 80mA which will drain a battery in 2-3 weeks. Sometimes I will drive my truck for a couple weeks at a time in the winter, so even that was unacceptable.
The Intrigue BCM applies power to the hot wire of the mirror as soon as the door is opened and will cut power after 10-minutes if the ignition is off, so that allows the domelights and map lights to appear to function normally. You can't sit in a non-running car with the map lights on for longer than 10-minutes because they'll shut off.
Interestingly, the mirrors predate low cost white LEDs, so they have a bunch of amber and blue LEDs whose light blends together to simulate yellowish incandescent light. It's actually pretty cool.
