Could be the heater core. Next time it is wet take a finger and push it into the wet spot and taste it, then spit it our. If it tastes sweet it is most likely antifreeze and the heater core.
If it is just water it will have no taste and it is most likely what the dealer said.
Why taste it yourself? Find a stray cat, cats love antifreeze they'll lick it up like nobodys buisiness if it is antifreeze.:lol:
Actually before you dig into the truck take it back to the dealer and get management involved. Let them know this is the second visit for this problem and request to talk to the service manager (not a writer).
Round up the documentation from the first trip.
Dont get angry or loose your cool with dealership personnel, stay calm and talk peacefully to them.
If they arent willing to go above and beyond to solve the problem then request to see the dealership GM.
If they give you any static with this request then call the Regional manager.
General Motors also has a 1-800 trouble line if your still having problems.
If you work on the truck yourself taking it apart may give them reason to transfer blame for this issue to you, and possibly attempt to not cover it under warranty.
I'm not implying the dealership will be anything but helpful for this issue, but incase they arent this is the route recommended by a good friend of mine thats a GM for a local dealership.