It could be that the condensation drain line is blocked and backing up into the car - but; it is more likely that your problem is a result of a poorly designed cowl cover over the blower fan for your air conditioner. this is a VERY common problem on 97-03 Grand Prix's as well as Malibu's and Impalas through '04. The other side effect of this is that water draining into the blower fan WILL cause it to fail eventually - this solution eliminates that risk.
Here is the cowl (pictures from an '04 Impala, FYI, but are identical):
You will need to remove it by pulling the retaining clips up - there is 1 on each end, and then one towards the top that has a slot in it. Be careful to not break these or lose them.
Remove the cowl, and underneath you will see the blower fan:
Under the cowl you will see a rubber seal that is indicated by the arrow - this is the poorly designed "drip edge". You can either remove it - or leave it in place; the new diverter will accumulate any moisture captured by this seal and divert it properly.
Here is a picture of the part you need - which can be purchased from any GM dealership for about $4 - the GM part is #15794785. Most dealerships don't stock it - but they can order it for you.
It simply sits in place as noted by the arrows - you'll notice there is a long extension from the left side of the part - this is the "finger gauge" - you want this to be positioned as far towards the left (as your facing the motor, or passenger side) of the vehicle as possible.
Here is a close-up of the gauge to give you perspective on how it should be positioned:
This is a shot of the drain area that the new diverter will use - instead of dumping water into the blower fan like the old design.
Now - for the cowl to fit over the new diverter - you will need to trim a couple of pieces of plastic from the bottom of the cowl - as indicated:
You can simply use a utility knife and a pair of pliers to score and snap these pieces off.
What the underside of your cowl should look like after removing (lines indicate where plastic was removed)
No you can simply replace the cowl, and reinstall the retaining clips - and you are all done.
Hope this helps - be sure to rate it and comment! ;)
KC GTO Dad