The outside dimming mirror is wired to the inside mirror. When the inside mirror dims, the outside dims. The outside may dim quicker if you have dark tinted rear windows. The pin-out on the mirror (from the mirror mount going out) is:
1) 12V ignition
2) ground
3) reverse over-ride (12V when in reverse to clear the dim feature)
4) outside mirror + (1.2V when dimming)
5) outside mirror - (ground)
Make sure you are not blocking the sensor on the back of the mirror (facing towards the front of the vehicle). This is the ambient light sensor. Blocking it puts the mirror into night mode. Any light on the front of the mirror (glare sensor) will then darken it. Good luck.