Sorry can't get to my car right now.... its all covered up and has a bunch of ice/snow on it... I don't use it in the winter.
Looked up the service procedures and it appears that the plastic cover simply snaps off with a pull; it separates along the seam that runs around the mirror. Procedure says to pull off the front piece first while holding the mirror firmly. ... but, I'd sure hate to try it if its cold... my experience is that plastic trim pieces don't take kindly to being pried on when they're cold... they break instead. Also, I think these plastic pieces are really expensive...
Proceed at your own risk!
Sorry not more help.

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KenP

1996 S500 W140 Coupe