1999 Pontiac Grand Prix Water getting into trunk

This is a job for RTV Silicone Gasket Sealer. I'm familiar with two products from the Chrysler parts department but other car manufacturers are going to have the same stuff with their own names on them. Chrysler's black stuff stays more rubbery and it will not bond and seal if there is a film of oil on one of the parts. The gray stuff gets a little harder and will bond through a little oil, so we use it on transmission oil pans in case we miss a spot when we're cleaning them.

There's a lot of similar products at hardware stores and auto parts stores. For this application you don't have to pay extra for things like "oxygen sensor safe". These sealants give off fumes while they're curing, and some of those fumes could damage oxygen sensors.