1999 Volkswagen Passat wet under passenger front seat

If it is plain water than it could come from poor weather stripping around door window or windshield, it may also be from a clogged A/C drain tube which runs from the evaporator. The evaporator is found behind the passenger dash which is why water would be in that area. If it is actually coolant and not water it may be from a faulty heater core, again this is located behind the passenger side dash as well.