Water on passengers side and back

May be air conditioner stopped up running in grooves on front passenger side to back floor board

the drain hose has come off of evaporator housing. look for the hose that has come detached and reattach back onto drain nipple of evaporator.

Sounds like the outside drain is stopped up.
There should be a tube coming out of the firewall near the passenger floor.

Use a long Zip or Wire tie to run up it...

When the air conditioner is running, it forms condensation in the heater box, which is located under the dash board and quite difficult to access. fortunately, you don't have to get in there. The heater box has a drain tube that drains the water out of the front of the fire wall.In the engine compartment,look on the passenger side ,down low on the firewall, for the drain tube. Sometimes it's hard to see, but look closely, its probably covered with the same sound deadening as the rest of the fire wall. You may be able to stick a wire or similar item up into the tube. If not, it will take air pressure from an air blower to blow it out. You'll know when it's free when water starts coming out.

The usual problem when this happens is that the drain line from the AC condenser is plugged causing it to fill up with water which eventually finds a way out and drains onto the passenger side floor. It is fairly easy to fix but requires getting underneath the car on the front passenger side, locating the drain line, normally a flexible hose coming down through the frame roughly below the dashboard. and running something like a coat hanger up through the drain line to remove the blockage. Beware that water will start to run out the hose as the blockage is removed.
Some cars have this problem more frequently than others and you may have to repeat this procedure every year or so. When it starts dripping water on the passenger side again it's overdue.

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.

Most cars have the air conditioner drain on the passenger side just in front of the fire wall , if this becomes clogged it will back up and run out in the area you indicated. Look for the drain hose and make sure it is not kinked or plugged!

The problem you describe could be from condensation. On very humid days, I have seen some condensation from auto air conditioners blow through the ducts. If you run your A/C in the heat/defrost mode (it probably comes on by itself when you run the defroster), that could happen, and the water could be leaking down through the heater vent. It could be a sign that you're A/C unit is low on refrigerant, in which case, you'd need to get the system serviced.

A leaking door seal on the passenger's side could also cause that, but only if it was raining. If I assume that this is happening when it is not raining, the A/C is the only place water could be coming from. The only other, remote, possibility is windshield washer fluid, but you would probably notice that by its odor.

Hope helped (remember rated this help).

under car pass side just behind tire on firewall looking for tube that makes 90 degree bend make sure not clogged if still leaking check for cracks in the evap case

The heater/airconditioner box under the dash has a drain hose to allow condensation to drain when the air conditioner is running. If this hose gets plugged, rain may accumulate in this box also (if it's raining heavily), as it comes in through the fresh air intake.

Look on the engine side of the firewall, probably on the passenger side, for a hose coming through the firewall that is not connected to anything. It is usually short, has a 90 degree bend, and the open end is down. Make sure that the end of this hose is not plugged.