Hopefully you've already tried this:

I think the saved volume level for AC power is different than the saved volume level for DC power. After pulling my backup, never used, riocar off the shelf a week or two ago, loading up my tunes, and listening for a while at home, I freaked out when I brought it to the car and got no output.
The display indicated everything was operating fine .

Turns out the volume control was at negative infinity db....or whatever the lowest setting was!

-brendan