Thank you, Larry Fritz, for the Exhaust Brake circuit schematic from the 98 Patriot. This looks like it is very similar to the wiring in my 99 Contessa. I have taken the photo Larry sent me via phone text, and have annotated it with the location of the K8, K9, and K6 relays (Endurance Brake, ABS Retarder, and Cruise Control). This is going to help me a great deal in my troubleshooting.
P.S. I have modified the drawing to show the as-built wiring I have traced out so far in my 99 Contessa. There are a few significant differences with the Patriot, like the fact that the dash Exhaust Brake light is driven directly from the 205 line wire, and so does not actually indicate that the exhaust brake solenoid has actuated. I am still trying to find the physicall location of the "ALLISON VIM" relay, which actually controls the Pacbrake (exhaust brake).