Can be numerous reason, but first I raire the carpeting and undermatting. If the undermatting is wet, DISCARD, it will never dry. Not quite sure of your vehicles layout i.e. sunroof, etc. Look for a water leak ( water leak test ) from the front windshield , draining down to the floor board and carpet. Another spot could be an open rubber grommet in the firewall. If you do have a sunroof, is it a factory installed unit or aftermarket. Different type of installations. A leaking sunroof can drain water all the way down the windshield pillar to the floor carpets. Only way to really find the problem is 1 person in the car, and 1 person spraying water.
Could be the drivers.
A bad radio speaker
short circuit in the circuit
seal. expensive
motor mount
transmission leak in reverse only
Partially clogged fuel filter or the pump is failing.
Probably the blower motor, wiring to it or control switch
start with checking thermostat
You might need transmission fluid !?!?
car door light stay on
air mixing door actuator motor behind glove box