[shipfaced]

My '80SC with the factory option 4 speaker set-up was wired funky so I would check this first. The original Blaupunkt 2-channel head was wired: Left chnl=left front; rt chnl=rt frt, rt rear, left rear in series. Had to run new wire to rear speakers to get true 4 channel when I updated head (yes, this was a major PITA). Just something to verify while you are at it....