A/M radio

I suppose you could mount an FM converter anywhere. They are usually designed to hang under the dash, but the one I saw only had an on/off switch -- you could tuck it out of sight, anywhere you could reach in and hit the button.

Although, you'd probably get better sound out of one of those speaker docks for an iPod or smartphone. It could be powered via USB from a cigar lighter adapter. I don't know where it would sit, though.