WARNING: This page is about a different car, the 2003 Lexus LS 430, 2002 Lexus LS 430, and 2001 Lexus LS 430. However, it is still accessible from the selected car via links, so may be relevant.
For wiring diagram see Fig 1. For testing procedure see Fig 2.
Fig 1: Multiplex Communication Circuit Wiring Diagram