There is a drain tube that goes from underneath the gas tank cap to dispose of overfill. I have no idea if this passes through the trunk on your car or could be leaking into the trunk (But you would expect occasional gas odor in trunk if it were, no?).

Anyway, this thread - http://www.peachparts.com/shopforum/showthread.php?threadid=6834&highlight=fuel+drain+overflow - refers to the overflow line as passing through the trunk, being metal at both ends, and rubber or plastic in the middle. The thread also discusses the location of the flow-through ventilation doors or flaps by the rear bumpers. So there is a remote chance one of these could be a source of water leak if the most obvious choices don't work - maybe there is corrosion or impact damage around the doors or drain tube? Good luck.

__________________
John

2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)

Click to see 87 300E