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I recently got to troubleshoot the same problem in my '83 Porsche 944. In our weather the moisture from the wet floor condenses on the windows and fogs them up pretty good. And on really cold days, there's nothing worse than having to scrape ice of the outside and inside windows. Note also, that you may have a leak at the front of the car, even if just the rear seat floor is wet, due to the design of the floorpan.

Here's are the probable causes, from front to back:

1. Battery tray tends to rust/corrode. This is the most common cause ofsoaking wet floors on the passenger side. Check for pin holes, and later, larger visible holes under and around the battery, even if you don't see signs of rust. If you've had a new tray put in, make sure the drain hole is clear. The holes may not be visible so do a water test by plugging the drain hole, fillingthe traywith water, and checking inside under the passenger dash for drips or leaks. Alternatively, scrape to see if there is rust under the paint. Clear the drains on both sides until they freely drain. Fix pinholes with a roll on seam sealant, or serious holes with a new welded in tray. Prophylactically, clear all the leaves and stuff out of there, and make sure the drain hole is clear. Keep the area clean, and invest in a battery cover.

2. Lower windshield black plastic cover. (Black plastic cover above engine fan) It has a tendency to separate from the top seal area and cracks due to age. When the seal is shot, water will get underneath and run down the fresh blower discharge opening in the bottom behind the firewall. Use some black silicone windshield sealant to fix it up.

3. Clogged a/c condensation. Not very probable, but a clog could cause the collection tray to fill with water, which would then exit the front passenger footwell or via the glove compartment.

4. Outer rubber windshield seal. Pull the rubber trim from around windshield, which is just a top rubber seal. Clean all dirt and muck out. You may find there are small tears in the inner part of the seal. Just reseal the inner seal central groove with black windshield sealant. Then replace the top seal with sealant on it. Push it firmly into the groove and clean up any excess as you go. Be sure to pull the top seal completely out and check all of the inner seal. Shouldn't take more than 30 minutes.

5. Engine compartment cable seals. Less probable. Check that there aren't any cable holes with faulty seals, or other holes in the firewall which might be near areas prone to collecting water.

6. Floor pan cracks. When you drive the car in the rain, water splashed up off the road penetrates the cracks and soaks the carpet from the underside. However, it never occurs when you wash the car or if the car is just sitting in the rain without being driven. It's often mistaken for a battery tray or windshield seal leak. Pull back the carpet and check the condition of the floor pans on the wet side of the car, and check from the exterior if possible.

7. Bad door seals or rear-window seals. If there is a gap at the back top of the door and dried outer door seals, water can get into the car. Also if the outer bottom seal on the door window is messed up, water can get into the door cavity. This should drain out the bottom but the rather small drains get clogged too easily. This forces the water into the car at the bottom of the door panels. Use the paper currency check. Close the door on a bank note and see if you can pull it out. Repeat around the door. I found that the top rear corner to be "loose". Water will build up in the door jamb area between the door seal and the inside sill then spills over into the car.

8. Sunroofgasket and/or drains. This is probably the second leading cause of wet floors, as water drains down under the headliner. The gasketmay leaka minimal amount of water, but not more than the drains can handle. Check the condition of the gasket for serious abnormalities. Then with the sunroof completely removed, run a straight length of fully-extended, wire coat hanger (or better yet, stiff weed-whacker cord), down all four drain holes, at each corner of the inner sunroof recess. If the wire stops, or cannot be run to the end of its' length- the drain path is clogged with either dirt, leaves, or other debris. This must be removed, in order to avoid being saturated. Do whatever it takes to remove the obstructions, and verify removal by running hot water entirely through all four drains. Make sure all attached hoses are not damaged, and lead to exiting outside the car. (Compressed air can sometimes work to free obstructions if you have enough psi, and a rubber funnel nozzle; but may just compact the debris.) If the holes are clear, ensure that the drain pipes are good and tightly secured. Also, the sunroof drain tubes may be clear but if the rubber roof seal clogs the tube then the water on that side of the car then can seep thru the seal and into your cars interior. There are 4 adjuster screws ( phillips head) one at each corner try adjusting the problem one upward to be sure that the roof seal is not closing off the drain tube opening

9. Sunroof isn't closed securely. Sure, it sounds silly, but check anyway. It's a cheap fix.

10. Rocker panel drains. Located on the pinch weld. Make sure they are clear because water will fill the rocker and then spill out of the holes, and plugs on the inner rocker. You can use a small screwdriver to clean these out, they may be crushed shut. I have seen this happen if the car was ever on a frame machine, to repair structural damage. They will clamp to the pinch welds and some times crush the drains.

11. Rear wheel arch rust.Both at the leading edges of the rear wheel wells and also take off the wheels and look behind them at the sides of the wheel arch where the wheel arch panel is welded to the front part of the kickup of the chassis member. They rust there and water could flow down into the rear of the cabin.

12. Fuel cap drain tube hole. There is a drain hole by the fuel cap that has a tiny bit of hose that may disconnect, allowing water down into the body of the car. Less probable.

13. Rear hatch frame. The 944 is equipped with a glass hatch that is surrounded by an aluminum frame. This frame is supported by 2 gas pressurized struts (attached at each rear corners) and two hinges at the front. When the hatch is in the closed position the strut place "reward" tension on the glass. Over time this tension cause the glass to be pulled loose from the forward portion of the frame. This will ultimately break the seal and cause water to enter the vehicle. The tension on the hatch glass can cause it to separate from the metal, usually at the top of the hatch. Closely inspect the union of the glass and metal frame, and seal with windshield sealant, or if it is bad, have the glass reseated. Or, reseat the glass yourself: http://www.pelicanparts.com/techarticles/944_hatch_glass_repair/944_hatch_glass_repair2.htm

14. Rear hatch seal. Also, the rubber rear hatch seal can be worn and/or cracked, or may be compressed and no longer make a seal. It is easily replaced by aftermarket parts.

15. Rear hatch drains. Much of the water that drains around the perimeter of the hatch drains down to the hatch locking bolts at the bottom of the hatch. The bolt holes (the two cone shaped ones at the rearmost part of the hatch frame, have drain holes in the bottom of them. These arefitted with 3/8" white/tan drain pipe which should run to the rear storage compartments, and then out holes to the exterior of the car. If your rear storage compartments are full of water, these hoses may be clogged, may have become detatched, or may run to the rear storage compartment, but not run all the way outside. These lines need to be removed to access the rear light assembly, and should be re-attached after working on the taillights.

16. If none of the above solve your problem, your girlfriend may be sneaking out to the car in the middle of the night and dumping pitchers of water onto the floor, just to keep you occupied and out of her hair. Or, the porsche gods have it out for you. Either way, there isn't much of a solution.