Sorry for the delay, out of town.

There is a thin plastic trim piece, U-shaped, which follows the line of the mirror edge (upper, outer, and lower edges). It is pried out with a plastic wedge or something which won't mar the finish. Once that is out, the back is clipped in, how so evident with a look-see.

I know the 300's and the 400's are not identical. I thought all the 300's had black backs. Your mileage may vary....

__________________
Peter
1985 300TD 4-speed 212K
1992 400E 343K
2001 E320 72K