Oh... DUH... I forgot the most important thing!!!!

Are you running the latest hijack? If you are, there's a setting you need to select in its main menu. The setting is only visible when the player is in car mode. He added it in a recent version. I forget its exact name, but it controls whether or not to launch the player app with -s- so that the player doesn't grab the serial port.

If that's what it turns out to be, I will point Mark to this thread as proof that he should have had that option default to the no-serial behavior.