I believe GMC used a positive ground system at the time. If that is true then one thing should be noted. The original radio had a mechanical vibrator which could be used in eithe polarity. However the solid state vibrators are polarity specific.
It’s sad because a solid state vibrator could easily be built to work on both polarities as well But I get why the original circuit designers assumed a negative ground it makes the design cheaper. The synchronous mechanical vibrators in the models 48 and 490 GM Autoradio did care about polarity, however you can rotate the vibrator in its socket per the warning label in the radio and it can be configured for either ground polarity to use in Chevy or GM trucks. Models 500 and onward changed to non synchronous vibrators like the USA radios and then worked on either polarity without user intervention.
I only restore radios with the mechanical vibrators so they are original and work on both polarities, the hum is not noticeable if the vibrator is in perfect working condition. -s
My GMC has been converted to negative ground. I bought that very nice looking 986067 on eBay yesterday. Recording to the listing, it had been restored 20 years ago never used and kept dry storage. Hopefully that is correct. Fortunately, it seems that we have an expert stovebolter on this specific model!
