I have a 1980 300cd with the same moisture problem in my trunk. Check under the plastic trunk liner on both sides to see if the drain is clogged. The liners come out pretty easy, and if the drains are clogged you might have standing water. As far as the rust goes, check the bottom of the doors where the weather stripping is attatched. Also, check behind the passenger side kick panel. Dirt can get trapped up there behind the front tire and cause rust as it did in mine. Next time it rains, check under the passenger side carpet to see if it's wet. andy