Light Control Rheostat
WARNING: This page is about a different car, the 2003 Lexus RX 300 and 2002 Lexus RX 300. However, it is still accessible from the selected car via links, so may be relevant.
Gradually, turn the rheostat knob from the bright side to dark side and check that the resistance decreases from 10 k ohms to 0 ohms between terminal 4 and 5 (rheostat knob turned to clockwise). See Fig 1. If operation is not as specified, replace the rheostat light control.