Thanks - I had already decided to try that and just did so. Still seeing the same behavior. I'm not sure why this is so difficult - I'm grounding the wires in the same way. I see it work until I'm moving over 10 mph or so and then the warning message, etc.
In my older Pioneer unit I just ground the parking brake wire to something metal and voila.
Oh well, guess it's not really the end of the world...