As i'm installing a new carpet in my 1997 Monte i pull the rear bottom cushion and there's 1/2 cup of clear water on the floor of the frame on the left and right side. I went ahead and pulled the back cushion off and found that the top of the shock tower supports located in the trunk where really rusted allowing rain water to trickle down onto the rear floor. Turns out the rear fender joints in the wheel well was allowing rain water from the rotation of the tires to enter into the shock tower supports. I guess after 12 years it ate thru. It took 6 hrs. of wire wheeling and wire brushing, a couple of cans of rubberized undercoating. I plan to wrap this fender seam with some HVAC putty when my undercoating is dry. Might want to check your supports in the trunk ya'll ........Tom