[dmjlambert]

If you don't want to get "that much into it", the grey wire that goes outside of the AM radio case and travels to the light bulb holder in the back of the case is powered on when the radio is turned on. You could tap into that wire to power your bluetooth amplifier, and just tuck the bluetooth amplifier up into the dash somewhere hidden and leave it on and set to whatever max volume you are comfortable with. Set the volume on your phone when playing through bluetooth. The radio on-off control would give you the ability to instantly turn off the amp instead of fumbling around for your phone. For bluetooth amps that have aux input, the aux input works when there is no bluetooth device connected. You could tap off the center terminal of the volume control on the car radio, and send that signal into the aux input of the external bluetooth amp, and have a functional and modern AM radio.