Seat Belt Warning Occupant Detection Sensor
WARNING: This page is about a different car, the 2002 Toyota Avalon. However, it is still accessible from the selected car via links, so may be relevant.
Disconnect 2-pin Black electrical harness connector from occupant detection sensor located under passenger seat. Using an ohmmeter, check that continuity exists between occupant detection sensor terminals while depressing sensing pad. See Fig 1. If continuity is not as specified, replace occupant detection sensor.