No idea what's wrong but some ideas on what to do:
If you/previous owner ever keep/kept stuff under the seat, look for damaged wiring or damaged connectors.
If you had a moisture problem, look for a bum motor.
Use an ohmmeter to test the switch - unplug the switch and use the meter to check function. . Maybe it's just the switch.
check wiring diagram to find any splices (I don't think there are any) and if there are any, find them in the wiring and check them.