Not able to give a great step by step one because last time I did it was awhile ago. and the radios differ so much inside from radio to radio.
Last one I did was the 5 band EQ tape deck in my IROC. I found the Left and right audio signals coming from the tuner to the amplifier and severed them, got a small 6 conductor wire, and snaked it out and through the console to a ON-ON switch in the console. one position lets the radio pass through as usual, the other lets the bluetooth module interrupt and go to the amplifier instead. I stuffed the bluetooth module inside the radio itself and its powered by the power switch of the radio so its truly hands off. Been working well for awhile now.
So basically I broke out the right and left tuner from going into the amplifier, brought those along with the BT out left/right and another set of wires going back to the tuner so I can still keep factory functionality.