I just bought a 2004 Pontiac Grand Prix GTP. We had a pretty good rain come through the past couple days. When I would accelerate, I noticed water was raining down from in between the two dome/map lights, located toward the front-center of the headliner. I have a power moonroof, so I was thinking there could be a problem with that, but I also have an antenna for OnStar which is almost directly above the dome lights.

Does anyone have any ideas what the common reasons could be for this leak? FYI, I'm new to GM cars.

Thanks in advance!

In all likelihood, one or more of the sunroof drain tubes is disconnected from the sunroof. There is one drain tube in each corner of the sunroof. The front ones go down the A pillars and exit into the rear of the front wheel wells, behind the fender liner. The rear ones exit down the D pillars into the front of the rear wheel well.

Slide the glass back, and one at a time, pour water into the forward corners of the sunroof housing. You should be able to see, and target, the drain holes. Next, put the glass into vent position and repeat for the rear drain holes. Each time you pour water, look for it to dump to the road in the wheel well areas as described. When you find one that doesn't dump to the road you probably found the water leak.

To fix it, you have to drop the headliner (just drop, not remove) and reconnect the tube to the fitting on the sunroof tray. Add a nylon tie strap or small gear clamp to each of the four connections to prevent it from happening again. Removing the A pillars, B pillars, visors and grag handles will allow you to drop the head liner.

yep pretty common problem the tube attachment from gm is bs. this guy tore his whole car apart fixing leaks you probably dont have to do that but he took some good pics of whats going on in there.
https://www.gmforum.com/2000-2005-90/help-my-car-taking-water-297561/

yeah very common check the drain tube one probably came unhooked

Thanks for the suggestions. The weather is supposed to be pretty nice tomorrow, so I may try to figure it out then. If I have time to work on it tomorrow, I will take pictures.

Dear Black04GTP and Everyone, I had the same problem with water leaking into my car through the dome light. I assumed that it had to do with the sunroof also. My son and I checked out the sunroof'* 4 drains and the seal around the sunroof and all seemed to be working correctly. After we went over, when I first noticed the leak and so on, we decided to check out the Onstar antenna. The top part of the antenna (nearest the sunroof) did not seal tight to the roof of the car; even though the rubber around the antenna seemed in good condition yet. We had to take apart the doom light and remove parts to get to the antenna. Even though we tightened it well it still didn't look much tighter fitting to the roof, so we put clear silicone under the antenna and then tightened it again (and of course, cleaned up any extra silicone from around the antenna). Under the antenna, there is a small hole in the roof where a part of the antenna fits into, and the silicone sealed in that area also. It has been three weeks since we fixed this, and endured a down poor of rain and snow melting, and I am very happy to say "the leak is gone". I hope this helps someone because I know that I was so frustrated with this leak. Have a great day!!!!! Ladiebug61 2/10/13

X2 for Onstar antannae. That was the cause. If you do not fix it, your roof sill at the windshield will begin to rust.