2008 C350 Mirror replacement
 

I recently purchased a 2008 C350 that was missing the left (drivers side) mirror glass. The mirror has the heating and auto dimming feature. I was shocked at the price the dealer wanted for the replacement glass but found it online for much less. I received the mirro glass and the backing plate that attaches to the mirror motor. My question is does the backing plate on the mirror just snap on to the mirror motor or is there some sort of retaining ring or snap that holds the mirror onto the motor? I don't want to take a chance of the mirror falling out because I'm missing a part. Any help is greatly appreciated.

on a mirror glass from the dealer. not sure what you mean by backing plate. but the glass itself does have a black backside which clips onto the motor with little tabs that click onto the motor.
start to attach the bottom of the glass first. then push on the rest of the glass. there are no other clips to hold on the mirror.