I would make sure your cooling system is full. I had a 99 Jeep in last week with the same problem. The customer said the system was full. I put 2 quarts in the radiator and it went away. I have also seen the holes blocked for the vents under the wiper cowel and water was stuck in the wiper linkage area
I recentaly bought a 1998 Jeep GrandCherokee 5.9 with water sloshing around and water filling up the passenger side floorboard.I searched and found many answeres that did not work for me. BUT I finally found the cure. These ins. will be lenghty so please bare with me. first I had to pull the carpet passenger side back and split the ruber cover that covered the drain outlet on passenger side firewall just below heater box. The drain cover will have a plug in it . The cover is round about the size of a 50 cent piece. Pull the plug out and see if water drains out, If not there is an obstruction from that point up to the evaporator[ heater box]. I drilled a half inch hole just to the left of the plug hole and carefully ran a coat hanger slowely up drain to clear drain. This was not my problem. I than ran a long coat hanger through the hole where the plug hole is to clear any obstruction, for me this was still not the problem. I still was getting lots of water in floorboard? Than I found the Problem. If you run your fingers around behind the round drain cap that has the plug in it that connects to the fire wall, you will notice there is a seam where two pieces of plastic pipe connects. If you can, run your A/C unit and watch to see where the water is going. For me It was seeping from where the two pipes connected at the firewall. The one pipe exits through the firewall. I took some auto weather strippin sealer that comes in a string and worked it into the seam and kept watching until all the leak had stopped. Next you will want to check the drain out side of the car where the water exits. on my 1998 Jeep 5.9 the water exits through the frame just below the passenger side front door through the frame. I also took a coat hanger and cleaned the passage way. I know this was long but I hope this will help you. It took me a lot of effort to finally fix this problem.
I have found that my heater coli is constantly on ,causing the car to be in defrost which makes more water. I removed the fuse and run in recirculate this creates less moisture,haven't fix the sloshing yet but close. PS my son has 2002 Jeep and his heater is on all the time?
This is a common problem with similar year jeeps. Cut the rubber from around the firewall and the black circle which is the drain under the passenger dash. Use a small screw driver or a nail to scrape the black foam from behind the drain and the fire wall. It will most likely crumble away. Spray the area with brake cleaner run the car with the ac off for awhile, the heat on the firewall will dry the area. Must be dry. , Then squirt alot of black silicone gasket maker in-between the drain and the wall. Make a smooth layer with your finger all the way around. Use alot, Its not pretty but it works and will never be scene anyways Let dry for at least an hour before driving and do not use AC for 24 hours. This is from a mechanic friend who has seen this numerous times. $6.00 fix or hundreds for a new seal. Caused by the heat breaking down the seal. Its not clogged and u will not find the hole under the car anyways. Its in the frame.