Last week after I had to drive through some really heavy rain, I noticed that the carpeting on the rear floor board behind the front pasenger seat was damp - not saturated, just pretty damp. I donlt have a sunroof, but all the seals around the doors and trunk are fine and there are no leaks...I checked them really good!

I looked inside the rear wheel well and there right above the back tire is a section of that original undercoating material that cracked off. I'm assuming that this might be the culprit. Regardless, it does have to be repaired.

I cleaned the area as best I could and then sprayed some heavy duty rubberized undercoating in and around the spot (which is about 3 inches long and a half inch wide) for the time being. But since the original material that came off was originally put on pretty thick by GM, I need something to build up the area where the old stuff was to even the damaged area out and to fill it in better.

Anyone have a suggestion what material I can use that would be acceptable.

First thing I'd do is check the inside of the trunk to see if it goes all the way through. If it does, clean all the rubber off you just put on, and repair it inside and outside with fiberglass resin and roving (woven fiber cloth), then re-rubber.

The most likely place for a leak other than the sunroof for the back seat is the rear window seal. Where the rubber meets the glass at the bottom of the window, water can leak in. Run a thin bead of clear or black silicone caulking along this seam. That fixed my problem.

my buick and now my bonne have that same problem lol

Fiberglass is easy and fast, especially in that area, as you can't see the repair later. Including prep and cleanup (and fully cured), it'* only an hour, and about 10 bucks for the kit at any auto parts store. Get the kit with the resin, mixing tray, and fiberglass cloth.