It's documented in the installation guide for the Navigation Radio unit.

It's documented in the factory wiring diagram.


My 2004 (with aftermarket head unit) raises and lowers volume as speed changes (subtle).

I conclude that if signal from head unit goes through factory amp to factory speakers, and the signal from the body control module is working, the system raises and lowers volume in response to speed, and my car (2004 Coupe) does that.