I need some help designing an automatic switch that will switch the input from three different sources to one set of speakers. I have an FRS radio, and on board computer in my truck. I put the sound from these into my head unit via a home made din plug that switches my head unit to AUX mode by delivering 12 volts + via a manual switch. I want to have it so that I can switch manually between the computer input and the radio/cd player built into the head unit. It does this now. I want it to also switch the head unit to AUX and use the FRS radio input automatically when a radio message is received. I also want the FRS signal to overide the computer sound when I am in manual AUX mode. Any suggestions would be welcome. Thanks.