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Either your windshield or driver's side window has a major leak
plugged evaporator drain tube or the heater core has gone bad
the heater core is bad
Water from my ac is draining in side the floorboard where is the drain hose on a 99 Oldsmobile intrigue
check the seals in the door, window, and windshield on that side. put paper towels (or something to catch water) next time it rains, see if you can find a pattern of entry.
check around your window on that side sometimes they will separateand you will have to have it fixed
Passenger's side? Could be a leaking heater core
A damaged heater core will cause coolant or water to leak onto the passenger floorboard.
If it is water the hose that drains the a/c evaperator is broken, if it is engine coolant it is a bad heater core In my case ('90 apv), the tubing which supplies washer fluid to the rear wiper cracked and seeped out, soaking the carpet. It runs under the carpet along the driver side door frame. The only up side is that the fluid apparently contains something which keeps the mildew from growing.
If that water is engine coolant, the heater core is leaking.
FRONT SEATING AREA, DRIVER SIDE, UNDER DASH, MOUNTED DRIVER SIDE OF STEERING COLUMN. It is located under a removable panel on the left side of the floorboard on the driver's side (the 'kick panel') It is the vertical plastic panel.
Check to see if your floorboard is rusted through. If it is patch it.