I have a 1996 Grand Cherokee and when I run my

We had the exact same problem with our grand cherokkee. The drain goes through the firewall, under the carpet, behind the glove box and comes out inside the frame on the passenger side. It's virtually impossible to get to.
There is a way to fix the problem. You can get a repair kit from your dealer. You have to pull back the carpet, which mean taking the interior side wall and the door threshold off. You then have to drill a hole in the vent housing, then using a small parts grabber reach in and pull out the debris that has built up, next you let the system drain (a small flexable bucket help for catching the water that comes in), then install the supplied plug. If my memory serves me right the kit came with fairly good instructions.