I have an AM radio that plays on 6 or 12 volts neg. or pos. ground. It has switches to select which one. I tested it & it plays real good. It will really work good for a 6 volt truck & a 6 volt pos. ground GMC. $25 shipped. I have an original equipment Delco 6X9 speaker i used for testing. I will throw it in free except for shipping. It has a heavy magnet. It sounds real good.
They say money can't buy happiness. It can buy old Chevy trucks though. Same thing.