Relay To Power PA Speaker When CB Radio Is Powered

I have a CB radio with constant 12V going to it, it has a power knob on the unit itself to turn it on. I also have a PA speaker with a built in amplifier. It uses a positive and negative to power the speaker, and it uses 2 signal wires that go to a 3.5mm audio jack that plugs into the CB. I don't want constant 12V going to the PA because it will always have a draw, even when not in use. I'm trying to figure out if there's a way to make a relay that is triggered when I turn on the power knob of the CB radio, which would then supply power to the PA, and I'm drawing a blank. I also used a multimeter to test if the signal wires of the 3.5mm audio jack carry any voltage when the CB is flipped to PA mode and PPT is held on the mic, but it does not. I was hoping if that were the case I could trigger the relay with the audio signal by holding down PTT. I briefly considered running the trigger of the relay in series with the 12V power to the CB, and when the CB power knob is turned on it would complete the ground and trigger the relay? But doing that means I'm running ALL of the current of the CB through the coil first and would likely fry the relay right? If this is not possible then I suppose I could just go the route of powering the PA with ignition or a seperate switch.

You will have to go inside the CB and find the wire on the power switch. It will be easy to find. Open it up and post pictures of the volume knob/switch and I can point it out to you.