I don't have a GTO or LeMans YET! but I have had the exact same thing from a Jeep Cherokee. It would only occurr if it is rained on for a lont time, overnite for example and there was a lot of humidity and condensation. It would also do it if it snowed and again humidity/condensation....
It took me quite some time to figure out, I tore into the engine bay checking the blower motor and any seals on the firewall. I checked the windshield seal, I checked EVERYTHING! I even called Chrysler who was of no help.
I found the issue in the A/C system drain. Apparently there is some sort of tray the collects the drip-off condensation from the evaporator core I beleive... Anyhow it was clogged and all the condensation would fill the tray shortly after as I started the engine, and when I started to move it was a small stream of water running down off my blower motor housing and into the passenger floor then it was done (for the day). The next rain storm, same thing. Now the drain tube came through the firewall near the blower motor under a canister. I blew compressed air in it, you could hear it blowing out into the passenger compartment area. I havn't had trouble since.
That was a Jeep, but it could be rather universal issue, I talked to a reputable repair shop here locally, (Smith Garage) They said they have seen this on about everything with A/C. It may help you with your issue.
Goodluck!
