I'm working on repairing some rust problems on my '87 300SDL. I've
removed the cladding from the rocker panels and wheelwells without any
real problem.

But when I removed the plastic trim panels from the doors, the rear
most retainer was a problem. It pulled out of the trim panel
completely. I thought that it was a screw but they don't appear to
unscrew. Should I look into removing the interior door panel to see if
the things are bolts? Any suggestions?

The car looks quite nekkid without the plastic trim panels <G> Will be
doing a small fibreglass repair on trunk but that is another set of
problems by itself.