Jim,
This is a case where bus specific information is not required. What Bill needs is a wiring diagram or even a relay part number and an associated diagram to suit his needs.
Where Bill opts to pick up his 12V and his 12V or 24V signal is entirely up to him because only Bill at this point knows to what extent he wants to fish wires or where he wants to locate his controller and relay.
Bill could locate the controller in the big electric box in the rear (but lose the opportunity to be able to adjust it as needed) or he could opt to locate it at the driver's area. Likelwise the relay can be located where convenient for his purposes. He can get signal power from the brake light switch if he wants to get it from the front, or he can pick it up from the rear from such sources as a brake light wire or even from the signal circuit from the brake light relay.
Where and how is unimportant. The only thing Bill needs now is a wiring diagram and maybe a part number for a relay.