If it's the whole mirror assembly, you'll have to take the door panel off
and tighten the bolts from the inside.

-Stan

The clip came off of my '97 passenger side mirror and I could see
that the groove the clip fits into was very shallow. Not much to hold the
clip in. I fixed mine by using a Dremel cut off wheel and going around the
groove to deepen it just alittle bit. It was a bitch to get the spring clip
back into the groove but I haven't had any more problems with it since the
repair
two years ago.
I used a threaded rod, some nuts and washers to compress the
whole thing until the clip went back into the newly deepened groove.
I used a large socket on top that was large enough to go around
and push down on the clip as it was tightened. Hardest part was
finding all the stuff in my junk drawer and finding a socket short enough.

'97 C5
Gerry