There are two sensors used to dim the mirror. One senses ambient light in front of it, and one senses light from behind the vehicle. If you want it to be dim, you only need to block the front one. Blocking the rear is not needed. If light is present on both sides, it doesn't dim. It has to see darkness on the front and light on the back to dim down. The sensors are simply inputs, the voltage generated is used to tell a printed circuit if it needs to dim the mirror or not, it does not power the mirror at all.
2011 GT 6MT (Cookie Monster) - The 1/4 life crisis. Brembos, 401A, drop top, Grabber Blue.
1974 Dodge Challenger - The money pit. 408 SB stroker, Megasquirt 2 EFI, 904TF, 3.91 gears. 16MPG and 520HP goodness
2004 Hyundai Tiburon - The winter beater.