Here are the parts I used (you can use similar products):

Audiovox FM100A FM Modulator with IST2 Isolation Transformer
Comes with an on/off switch and a ground loop isolator for the signal wires. I mounted the switch into the back of the bottom shelf.

Fusion MS-BT100 Bluetooth Dongle (music only, not for calls)
This doesn't seem to clip the BT signal too badly. Music still sounds full. Wired this into the FM Modulator switch.

Audiophile CMOY headphone amplifier
Cut off the 9v battery connector, drilled a hole in the side, used a grommet, and wired this into the switch as well. I set the volume at about 70%.

Fakra antenna adapters
http://www.discountcarstereo.com/BAA2223.html

It won't give you CD-quality audio, but it's better than nothing and now I can stream Pandora through the car whenever I happen to have the radio on.

I was able to mount the FM modulator in the space behind the stereo (on the trans tunnel, covered by the carpet pieces) and everything else went under and behind the shelves under the stereo.