Best location for OEM bluetooth module?

It's personal preference really...

- The real OEM Audi way is under the seat.

- Sometimes you can squeeze it behind he headunit but it puts unnecessary pressure on things.

- Another place. Remove the lower dash under the steering wheel and you can hide and cable tie it away under there.
 
Nope. But I guess you could try there.

Its the panel directly below the steering wheel. Couple of 8mm bolts on the bottom, one on the side Near the fuse board and then clips along the top near the steering wheel.