I just found an old stereo in my garage that is a perfect fit for this project- it's an Insignia something iPod dock (Best Buy's house brand.) I always ran it off 120v mains, but it also can run off C-cell batteries, at 6v! I gutted the system last night, and will try to update this log + post pictures. I'm also ordering a 12v to 6v converter from ebay- more on that when it arrives!