Circuit Description

WARNING: This page is about a different car, the 2005 Lexus RX 330. However, it is still accessible from the selected car via links, so may be relevant.

The rear defogger & mirror heater switch on the multi-display inputs a request signal to the A/C amplifier (ECU). Then, the A/C amplifier turns ON the RR-DEF relay built into the J/B No. 2 in response to the input, thus switching ON the rear defogger. (The deicer and mirror heater also turns ON at the same time)
The RR-DEF relay is built into the J/B No. 2. For inspection, use the terminals in the J/B No. 2.
If RR-DEF relay is malfunctioning, replace the J/B No. 2.