Occupant Detection Sensor
WARNING: This page is about a different car, the 2001 Lexus LX 470. However, it is still accessible from the selected car via links, so may be relevant.
Disconnect sensor harness connector, located under passenger's seat. Using DVOM, check for continuity between sensor connector terminals No. 1 and 2. Continuity should be present only when pressing on sensor. If continuity is not as specified, replace occupant detection sensor.