What S500 are you talking about - W140 or W220? In any car there are more than 3 wires for the mirror.
Here are the wires for W140 as of 97:
Black/Red -- +12V
Brown -- Ground
Pink/Black - output1 for driver side mirror
Pink/Red - output2 for driver side mirror
Pink/Green -- Dome Light ON
Grey/Yellow - Reverse

Regards, Mike