if you have the old mirror, the back comes off (just did this on my wife's 400) and you can swap the two covers. In fact the mirror I got from the factory didn't have a back, had to use the old one.

Might be different for the 300. Check and see.

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