Why would a car have water under the passenger seat?

Why would a car have water under the passenger seat?
There are a lot of possibilities. It could be anything from a baddoor seal which lets in water while driving when it's wet outsideto a leaking heater core.

To check for door leakage, simply open the door and look around theedges and underneath. If the rubber seal is damaged or missing, itmay be the cause of the water intrusion. (Remember that the watermay be coming from behind the seat too, depending on how level yourfloor is. So check both doors for seal damage.)

If this is the problem, you can get a replacement or you may beable to buy some self adhesive foam weather stripping and, afterthoroughly cleaning the surface around the missing rubber seal, usethe weather stripping to fill the gaps. This should solve theproblem.

If this is not the case, you can look under the car and see ifthere is any panel damage to the floor. A puncture or rust holesunder the seat may let in water from wet roadways.

To fix this, you can find some kind of filler (liquid electrician'stape, a spray-on rubber coating, expanding insulation foam,fiberglass, etc) to fill the damage. Thoroughly clean the area andapply the filler (if the hole is large, you will probably use theinsulating foam and/or the fiberglass to make sure it is sealedproperly.) After it dries, everything should be fine.

This is the harder of my suggestions, but is often the actualculprit. If the water is in the carpet in front of the passengerseat too, you may have a leaky heater core or hoses.

Sniff the rug. Is there an odd, almost fishy smell or is the watera rusty color? These are other signs that the heater core may bethe problem.

You can often see the core under the dashboard without having totake anything off of the car. If the carpet is dry in front of theseat it could still be the core because most carpets have a plasticbacking which could hide the water until it pools under the seat.

Wad up some paper and put it under the core (if you can see it).Check it after while a while, maybe even drive the car for a fewminutes. If there is water anywhere on the wad of paper, you knowthat your core or hoses are leaking.

You might be able to fix the core yourself, depending on how it isinstalled. Hoses might be removable without removing the core ifthey are the problem, but it is too difficult to explain anyfurther the repair of a heater core since there are so many stylesand installations.

Another, yet much less likely possibility is that there may bedamage to the door frame, door post, or roof of your car which maybe allowing water to leak down from above and get under the seat.