I've been searching without luck for information on adding an auxiliary input to a vintage, in my case 1975 Ford AM/FM car radio. All I seem to be able to uncover is info about FM transmitters, FM modulators, cassettes with plugs hanging out and using the remote CD changer input. Non of these are really what I'm looking for since the radio doesn't have a cassette or CD player, and FM transmitters and modulators typically have very poor sound quality. I'd like to learn how to hard wire directly into the circuit. Any help would be appreciated.