I wasn't sure where to post this, but it IS electrical, so

Check out woodys custom shop on evilbay. They sell multiple different hidden audio setups that are very nice.
2 channel mp3
4 channel Mp3
4 channel mp3 and bluetooth with preamp outputs
4 channel AM/FM bluetooth, mp3 with preamp outputs.

The box is about 3"×5" x about 1" can hide it anywhere.