i had the same problem. what i did to fix was simple. it came with water in the trunk when i bought it. first thing to replace is the gasket aroound the tailights. secondly if you have the trim water can leak through there. i fixed the lights but i still had some water come in when it rained. so i simply went into the trunk and put hot glue on each spot where the trim is plugged into. i no longer have leaks. Oh yea check for the fender rust. it might be a good idea to fix that.

jsambe,
Check the tail lights...they can often be thightened down... sometimes the gaskets around the lights ($15-$18 each) need to be replaced due to aging (become brittle, dry rot, etc.) Then, move to the trunk seal and lid. Pull up the carpet and remove any plastic to look for rust and rot... especially in the wells. Hope all goes smoothly. Peace.

The trunk in my 240 is also somewhat wet. I did notice that most of the moisture was coming from condensation dripping from the trunk lid. I think that the seal must be bad, allowing moisture to enter.