Thats so cool, how did you get the station preset buttons to manipulate the bluetooth board? They have some pretty stout single board amps available that would fit in there no problem.
I removed the "tuner" part of the tuner assembly and installed microswitches that the pushbuttons hit. The bluetooth board has a harness for the inputs that originally had another separate pcb with tact switches on it. The BT module I used did not have an on board amp so the outputs went to the inputs of the amp module. Pretty simple wiring really. The hardest part is finding room to mount everything.