Water is leaking into the passenger side cabin under the dash...why?

You may have a plugged AC water condensation drain hole, it drains under the car at the base of the engine bulkhead on the passenger's side.

I have 2005 mustang and had the same problem 4 yrs after I bought my .Mustang when it was still under the warranty I took it to the dealer where I purchased vehicle.They charged me $150.00 to change the cabin air filter they said the old one was blocked with leaves and suggested not to park vehicle under trees which is kind of hard to do when I live on a street with a lot of trees. Everything was fine until 2 years later it started leaking in passenger side again.Pulled cabin air filter saw no leaves.Of course now warranty ran out .I went on a mustang web forum to find there were other people with same problem and not just a mustang problem there were other makes and models having same problems .One person fixed the problem. Here is the fix. Pop your hood look under where the cowl grill sits on the right and left side there is a rubber drain on each side( hard to see)so feel with your fingers check to see if they are blocked with dirt or leaves if so clean them so water can flow through them when it rains .That is what was causing water to come in on my passenger side right underneath the glove box in my mustang now every few months I check to see if they are blocked I have not had anymore problems with water coming in on passenger side . Sad thing is when i purchased my mustang i wish the dealer would have told me not to park my car under trees so i wouldn\'t have a water I would have thought twice about buying a car that you can\'t park under trees because I live in an area where there is a whole lot of trees.Thanks a lot Ford for designing such a crappy car !!!! Good luck I hope this fixes your water problem.

on the passenger side? if it is the passenger side is your heat exchanger carrying the coolant that is used to regulate the cabin temperature. it is under the dash. Usually what happens is that the condenser (heat exchanger) the drain hose coming thru the firewall inside the engine compartment (passenger side) clogs not letting drain the water removed from the cabin during the AC cooling process. then the water start to bleed thru the joints of the box enclosing the heat exchanger. the solution is to blow or vacuum the protruding drain hose to clear the water path.

Now if the soaking carpet smell like coolant the problem is different.

The drain for the water coming from the ac is clogged. Clean it up. It's easier to do that from underneath the car - the drain is located on the firewall.

the hose under the dash that is connected to the AC condenser has come off & needs to be put back on it goes from the AC condenser through the floor....hope this helps.....cheers

The drain for your A/C may be clogged causing the water to accumulate. You should be able to locate the drain from under the vehicle on the pasenger side, perhaps there is an accumulation of debri such as leaves for example. Also, sunroof drain may also be blocked. I can say I have seen this problem before, and its a good place to start. Good luck

You may be right about the sun roof. It should have 4 tubes to drain water that comes in around the glass. Sometimes the tubes fall off letting the water come into the cabin. The horn may be the part in the wheel. Apply it at different angles and it might start working. Sometimes they get rusty. I think your cabin filter would be under the hood near the firewall on the passenger side with a plastic cover.

Look under the passenger side of the dash, behind the carpet. Feel the back side of the carpet for dampness. If so, try this: In the same area, look for a black rubber hose that comes from the heater plastic box, and pokes thru the firewall. See if it is plugged up. This is the drain pipe for the AC box, and the cowl panel if the side drains are plugged up. Try that, and please get back to me.

Having just repaired this problem in a 1998 Buick Regal, I can tell you that fixing the water leak is actually quite easy. It took me quite a while to track down the source of water, but I have discovered that it comes through the cabin air filter. There is a seal on the passenger side of the cowl (A two piece unit, black plastic, against the windshield) that seals it to the windshield to prevent water infiltration. It is very small and somewhat hard to miss. I took the cowl off to change the filter when I discovered this.

You can make a diverter to go over the air filter under the cowl, or even easier is to use some clear RTV silicone (avalible at any automotive and most hardware stores) and put a good bead on the old seal. When you re-install the plastic cowl, be carefull where it goes against the windshield to make sure you don't "smear" the silicone. After you re-install the plastic retainer hardware, smooth out the bead and clean up the excess silicone(a wet finger and some old cardboard works great). Let it dry and clean up the water in the passenger compartment.

Cleaning up the water is the hard part. Removing the plastic trim around the door (if you grasp the trip peices firmly they just pull up off some metal clips,but they can be quite stubborn) you will notice the carpet just pulls up. At the bottom front corner of the front doors the carpet has a plastic hook that is around one of the trim clips. Lift it off that and the rest of the carpet towards the firewall just pulls up. If it has been a long problem, your carpet may be moldy. Also, the carpet edges and some of the more hidden panel clips are very sharp and may cut you while you are poking around, so be carefull.

Since the water entered through the air filter and right over the fan, you will at minimum want to replace the filter. I had mold and moss growing on mine. My fan also squeels and chirps now, so I know it got wet. Not much to do about that but replace it or carefully lubricate the motor.

In your question you mentioned a door underneath the filter not closing. On my '98 Buick Regal there is no door or valve of any sort. The filter sits right above the fan blades. It is the cowel's job to prevent water from entering this part of the vehicle. I hope this helps you fix your problem.

If you notice a 'sweet' smell, or windows tend to fog up, it's your cars heater core leaking. The heater core is located under the dash, passenger side, it's like a mini radiator. The cars cooling system directs hot coolant to it so you can have heat. To change it is an involved procedure.
If there is no evidence of the anti-freeze smell or windows fogging up, and u have been using your air conditioner, it would be a blockage in the drain tube from the a/c. The end of the tube is located under the car on the passenger side , behind the front wheel. The blockage needs to be removed, and will solve the problem. Let me know what u find Curt