lets see the top of the button board.
Would your phone work if the Siri and answer/hangup/music on my iPhone was keyed up at the same time? you could do a rewire job to the buttons and have A for A radio B for B radio and mode for the phone Siri and answer/hangup/music .
So if the Siri,answer.hangup,music can be keyed up on the same button take the positive wire from B and connect it to the positive wire of mode and take one of the positive wires of A and connect it to positive of B where you took the first wire off. I think I got.