Yes this is an old thread but continuously relevant..
IF you can live without AM/FM, there are a number of really clever Bluetooth options these days.
This one from MTX fits into a standard square switch hole:
https://www.mtx.com/awbtsw-rocker-sw...er-and-control
Wetsounds sells a very similar one to the MTX and also has one that will fit into a round hole. Lighter replacement anyone?
https://www.wetsounds.com/product/WW-BT-VC-V2