I recently replaced the rear view mirror onj my 83 300SD. If your mirror is like mine, it is held in place by 2 pins with a spring between them. it looks something like this
<ZZZZZZZ>. To remove the mirror, pull on it moving it to the side. This should compress one of the pins & the mirror should come out.
To replace the new mirror, put one of the ends with the pins in place. Press on the other side and it should pop into place. I hope this is clear enough for you.
1999 New Beetle TDI
1958 Edsel Pacer