Update...
Thought I'd pass this on, may help someone else.
Got a mirror off fleabay for half of what GM wanted. Installed it and it doesn't dim either. Tried it with ignition on and off. I think it was with te key on and engine off I was able to get it to dim. So the mirror does work.
Most of the dimming operation is within the mirror seperate from the vehicle control. The only thing that isn't is the signal wire from the reverse lights. This turns off the dimming function when you put the vehicle into reverse so you can see out the back better. IMO, they could have left this feature out and saved money here. In fact I believe the money saved made up the difference for an OEM front cast iron differencial. I also prefer the manual dimming lever over this automatic crap.
Last week when I checked the voltage on this wire it showed 3 volts when the reverse lights are off and 14 volts when the reverse lights are on. So I disconnedted this wire and the mirror starts working. Plugged in the original and it dims too.
My opinion again... The 3 volts should not be high enough to turn off the mirror. Especialy when the high disabling voltage is 14 volts. This is poor engineering.
I'm keeping the new mirror. It dims faster and darker than the old one.
Hopes this helps someone.
