Audio Systems: 1970 ford am fm radio, fm stereo radio, speaker output

Question
Good morning, I have a 1970 ford am-fm stereo radio with an original 3 wire speaker output plug coming out of the radio, I want to hook up the speakers but am unsure of the wiring and do not want to blow the radio, the connecter is like the trailer connecters, one pin shielded in plastic the other 2 uncovered, common sense tells me the covered pin would be the common however I want to be sure, Please help, thanks in advance, Dave  

Answer
Your better to test with a multimeter. One of the three should be common, and should show common with the case of the radio. The other two should be front + and rear +. You could just test with a speaker too. With the speaker plugged in, you should be able to fade front to rear. If you plug th espeaker into both + leads, the fader won't work.

Kelly