Inspect Driver's Lumbar Support Switch Circuit

Disconnect the switch connector and connect the seat wire harness to the floor wire harness.
Inspect the connector on the wire harness side.

Fig 1: Identifying Terminals Of Driver's Lumbar Support Switch Connector (Wiring Harness Side)
Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002

DRIVER'S LUMBAR SUPPORT SWITCH CONNECTOR SPECIFIED CONDITION

Tester connection	Condition	Specified condition
3 - Ground	Constant	Continuity
4 - Ground	Constant	Battery positive voltage

If circuit is not as specified, inspect the circuits connected to other parts.