Leaking water After washing car or good amount of rain water leaks into car on passenger floor near firewall. ???

Four possible causes (may be more):
1. Leak at door or window/windshield (particularly after rain or car wash).
2. The heater core is leaking (water will be on the passenger side floor).
3. Blocked A/C condensation drain
4. Leaking sunroof or blocked drain (should be evidence of water on the A or B pillars, also after rain or car wash)).

Pick one ...

Look for places where two or more body panels meet. They're most likely secured together with spot welds and filled with sealant at the factory. You may have to look in the channels that are under trim surrounding a window or other glass. I had have 2013 Altima that pooled water in the rear seat floor area nearly 2 inches deep due to a poor seal between two panels in the engine compartment (firewall) just below the front windshield. Water followed wiring channels and contours in the floor pan (without getting carpet wet) to the lowest point - the back seat floor. So - don't overlook *anything* Good luck!

Hole in firewall rain leakin down and in. At least thats my prob. Some silicone should work

There is a drain tube that clogs up. It is on the firewall under the hood on the passenger side. You have to access it from under the car. Use a stiff wire and clean it out. Sometimes you can clean it with a water hose and sprayer. Just don't put a lot of water in it. Spray a little, let it drain, Do this till you don't see any mold coming out. If it clears out and you still have water issues. Replace the heater core.

Passenger side? May be coolant from a leaking heater core. Does the water smell sweet? If the A/C has been in use it could also be a clogged evaporator drain. The frost on it melts and needs to drain.

Has it been in the rain or is this after its washed?
Maybe a leak in the windshield or something along the firewall. Have one person inside the car and another with a water hose on slow. Starting low, move the water hose slowly along the firewall, near the wipers working up. The person on the inside will watch for water to start running in.

Ac hose leak?
leaking heater core?

Hint: it leaks when it RAINS

If it were related to the heater core or the AC drain, it would be leaking when it is not raining also, so those diagnoses are not logical.

The grand prix has a long history of floor leaks from seal failures around the air intake under the windshield cowling, where RAIN water runs in.

Use a garden hose to simulate rain and watch where the water enters the vehicle. Use newspaper on the floor to try and pin point the drip or leak area's.

Unless this happens in the rain as well, I would not worry about it. Likely in the car wash, far more water is coming in than the normal drain system can handle, therefore causing it to overflow. If you do have a drain obstruction, find the drain hose (small right angle hose on firewall passenger side) and roll it in your fingers to clear it.

Did you have the windshield replaced recently? This is typically caused by faulty installation of a windshield. Sometimes it can be fixed by adding windshield sealant (polyurethene, not silicone) where the windshield is adhered to the subframe - most likely along the bottom edge.

let's try something here and see if works,
get a garden water hose and put the most pressure near where you see leaking water and have someone inside the car look for it.

G