To better assist you please give me your engine size in liter and the 8th digit of your VIN.
Based off of what you have said, there is a Technical Service Bulletin out on this. I have attached it for your review.
Body/Interior - Headliner is Wet Around Sunroof
Bulletin No: 04-08-57-007B
Date: December 06, 2005
Headliner Wet Around Sun Roof (Modify Drain Hose)
2004-2005 Chevrolet Monte Carlo, Impala
2004-2005 Pontiac Grand Prix
This bulletin is being revised to add leak repair verification steps. Please discard Corporate Bulletin Number 04-08-57-007A (Section 08 - Body and Accessories).
Some customers may comment that the headliner is wet or that water is dripping out of the overhead reading lamp.
Both front corners of the sunroof have a drain hose running down the A pillar and end up down at the floor behind the lower kick pad on the left or right side. The end of the sunroof drain hoses may be laying flat against the foam at the base of the "A" pillar. This may cause a partially plugged drain hose. During a heavy rain, water that should be draining away may back up and overflow into the headliner at the front of the vehicle.
1. Remove the lower right hand and left hand lower kick pad.
2. After locating the hose, cut the end of the hose into a point.
3. Verify that the lower "A" pillar drains are open by pouring a cup of water in the bottom of the "A" pillar.
4. Check the drain hoses from the sunroof down to the bottom of the "A" pillar by using shop air.
5. Reinstall the kick pad.
Sunday, December 28th, 2008 AT 3:15 PM