The interior rear view mirror on the '98 C280 was floppy. I dug up this thread and removed the interior light cluster. You remove the interior light cluster because the mirror wiring connects to the light cluster.

With the mirror out and on the table I pried the two pieces of black plastic mirror surround apart using a small straight screwdriver. The snap closures at the top are the hardest to get apart, so start with the bottom. Be gentle, be firm.

Once apart, the two flat springs fell out AND two black plastic bits about 4 mm in size. Turns out these bits were spring retaining clips had broken off the mirror surround. There was no way to glue them back in.

The mirror surround is ABS plastic. I used some clear sealer (one could use any fast drying glue, or superglue) to stick the mirror to the black plastic surround, and the adjustment assembly to the mirror, and discarded the flat springs and broken little bits.

Hints… when I stuck the mirror to the smaller piece of surround, I put the sealer around the ends of the mirror where there were no electrical connections. I put some silicone dielectric grease on the mating surface of the larger black plastic mirror surround so I didn’t glue the surround together forever. Let it all cure, snap back together (remember the top snaps are troublesome) and VOILA no more floppy mirror.

HTH!