After a few heavy rains and leaks through the domelight, I finally took the time to drop the headliner and investigate. The water dripped into and almost filled the front cupholder. I have a 2004 GTP Comp G. Here's what I found:
I checked the Onstar Antenna mount, cited by a few on these forums as the culprit. On my 2004 found it was sealed fine without a drip of water.
I tested the front two drain tubes. Found the passenger side wasn't draining. I removed the rubber tube from the sunroof assembly, connected compressed air to clear debris, and heard a 'bubbling' sound--like when you blow air into a drink through a straw. However, the driver side was completely clear.
So, I followed the tube through the A pillar and found that it terminates in a little compartment near the bottom of the passenger side door, underneath the carpet where the passenger's right foot would be. This compartment was filled with water and was not draining. There was also water pooled under the passenger side floor.
I suspect that during a heavy rain, this compartment and the tube become filled with water, and the driver's side drain can't drain the water fast enough, and this water pools in the roof liner, exiting the dome light as this is the lowest point in the front of the car's interior.
I believe there is a drain hole somewhere as designed by GM. However, there was so much sound dampening insulation in there that it probably clogged the drain. The insulation looks like expanding foam, like "Great Stuff". So, I removed as much of this stuff as possible, removed the covers off the front right wheel well, drilled two holes in an inconspicuous place, and tested. The water drained as it is supposed to drain. I did this a few times to get as much of the little pieces of insulation from this compartment with a wetvac.
I'll post again after the next heavy rain, but I believe this will solve the problem many of us have with water coming from the dome light and/or pooling near the passenger side floor.
I hope this helps somebody here.
