water in the cabin Grand Prix
If you have a sun roof, the drain holes are probably plugged. If you have A/C, then it's probably the drain that is plugged. Find the drain tube and run a stiff piece of wire through it. That should clean out the debris.
If you're losing coolant too it could be a leaking heater core. Any fog on the windshield? If so I vote for the heater core. HTH. -Jim
Mine gets water inside if I park it facing up hill in rainy weather. I suspect it is coming in around a cowl vent somewhere, but since simply parking it level or facing downhill solves the problem I haven't bothered to look further. As for the possibility it is a heater core, simply determine if it is clear water or has coolant in it. If it doesn't taste, look, smell like antifreeze then your heater is not the culprit.
Looking for a Used Grand Prix in your area?
CarGurus has 3,701 nationwide Grand Prix listings starting at $879.
Search Pontiac Grand Prix Questions
Are you a UK consumer? CarGurus now has a discussion forum in the UK.
Pontiac Grand Prix Experts