Never, Ever Touch the Mirror.

Note damage on left side of frame
Behind this cover are clips at the top that are tiny and ridiculously weak.
It pains me to see anything neglected on a car that I own. If something can be improved, I generally get around to doing that, but in the case of the rear view mirror on an E320 Cabriolet, my advice is simple: DO NOT TOUCH IT.

This part is unique to the A124 Cabriolet and R129 Roadster, ridiculously fragile, and unreasonably expensive. ($650, last time I checked.)

If your windshield needs replacing, make sure to pull this part off yourself. Do not let the Safelite tech remove it because he will break it. Frankly, I'm not even sure I'd trust the Mercedes dealer to remove it properly. The issue is that there are a series of plastic clips on the inside of the rear cover that will snap under the pressure of a gentle breeze. Correction: these tiny clips could break just by looking at them too long. They are that fragile.

One of the reasons an owner would typically remove the mirror is to have the windshield changed. Another is to tighten a wobbly mirror. Over time, a nut on the inside of the mirror can loosen up and cause the mirror to vibrate at idle. This can be annoying, but I'm telling you that you are probably better off just living with it. Removing the mirror is easy enough, but re-installing it requires some force and just the slightest rub against the bulbous backside of the mirror casing can tug at the internal plastic clips hard enough to make them snap.

If you do have the unfortunate task of re-installing a mirror, make sure to apply a drop of oil at the points of contact so that it pops into place more easily. It takes a fair amount of force to push it back into place, so be prepared to do that by exerting pressure on the base and nowhere near the mirror, its thin frame, or even its more substantial backing.

Clean this part gingerly, adjust it to the night setting gently when needed, and try your best never to knock it while using a sun shade.