OK I've posted on this before but I had stated the wrong sound system which is the JBL and not the premium sound system as I previously thought. I have no sound and have already bought a replacement amplifier for this problem. I cannot find anywhere how the radio/w cassette powers up the amp. There is a control signal on the radio called "logic mute" that connects to the amp at a terminal called "mute control". I measure no toggling of this signal with my meter, I measure only millivolts. I considered that maybe the control signal is current instead of voltage. I got an answer on my other post saying that this signal is only for muting when starting the engine. If that is true then I have no idea how power to the amp is cut. Battery voltage to the B+ amplifier terminal is there. I have good audio signals getting to the line in's of the amp. Again switching out the amp did not fix. So I would like to know what is the characteristics of the logic mute control signal coming out of the radio or where can I find the information? It is not on the wiring diagram which you would think would be a great help to technicians if it were. I have attached the wiring diagram. BTW JBL, a great historical American company established in Los Angeles, just liquidated.