Can't find the source of interior leak

Some have reported the spoiler mounts in trunk? Someone also suggested door is not draining properly and then overspilling into car.

And this was reported. All Pontiac grand prix from 1999-2009 that leak water on passenger floor is from the following: The front cowl panel, [Where windshield wipers are] Has a air box inside the panel, That when built has a two piece construction that is sandwiched together at factory, and over time, summer heat, Icy winters etc, dries out and allows water to get into air box and drip out below dash on passenger side. To fix: Remove wiper arms, remove cowl screen, and you will notice a box looking square on passenger side, Clean out in and around box opening top. Must be clean and dry. Next use ether urethane windshield sealant, or silicone water proof sealer. Reseal all the way around box, [Where metal body meets air box.] Make sure to re seal all the way around air box and let dry overnite. You can buy ether sealants at your local automotive parts co.

It comes from the front cowl area. Open the hood and look for any lose rubber gromets on the firewall. Remove the under dash panels then pull the carpet back on both sides. Which side is wet by the firewall?It leaks in and rolls back to the rear behind the front seats. Have a helper spray down the firewall. You stay inside and look for waterleaking in. These leaks maybe from a split seam also. Remove the trunk carpet then get in the trunk with a flashlight. Let your helper flood the trunk out. You should see no light coming through anywhere. Check the l and r trunk hinge area. Then check for a leak at the wheatherstrip. Youll find them keep looking. Youll need to strip out the interior to dry the carpet to get rid of that smell.

Hey,

Great addition to this thread! Please feel free to help out on the site whenever it can add information that will help people.

Best, Ken

If the windshield that is leaking than best to remove windshield and reseal it
glass shop is your best bet

If it's coolant it is most likely the heater core, they may have done the repair incorrectly. Look at the heater core and see if it is leaking. A leaking waterpump would not leak into the cabin. Buy a Haynes manual and investigate the heater core.

Look under the hood, on the passenger side firewall, find the evaporator case, on the bottom of it there is a drain tube, these can block up with gunk. Use a can of compressed air like a computer duster and give it a few blasts, if this doesn't work, use a piece of wire to clear it.
You will need to peel the rug back remove the plug in the floor, and squeeze as much water as you can out of the rug, then get a fan and dry out the rest. Clean out the leaves and debris in the air intake under the black cowl, to remove that remove the wiper arms and the small clips along the edge of the cowl.

That's a big leak? What was it?