Roy,
If you could give a small wiring diagram for that arrangement, that would be great. I'm in the process of updating my TMCC USA Hudsons - Vanderbilt, Yellowbelly, 5344, etc. - to can motor and ERR Cruise, and want to keep the original (and more accurate) RailSounds in them.
I figured it would just be as easy as hooking up the ground, and serial data wire to the ERR Cruise Commander output like an ERR sound set does.