I've noticed something similar when upgrading Hijack versions a few times. The player would, upon being pulgged into the car, go immediately into suspend (heart-beat mode) and there would be no way to wake it up without pulling the player out and re-inserting it (rebooting.) This happened regardless of whether the car was on or off. It still worked fine on AC power though.

The fix was to re-load the original 2.0b11 software and then reapply the newer Hijack kernel on top of that. Fixed it every time.

Hope this helps.