GMC big rigs were all positive ground. The pickups simply followed suit to keep everything in the brand uniform. Back in the dark ages, commercial truck manufacturers preferred positive ground for some reason. In 1957, all the automotive makers standardized their electrical systems at 12V negative ground. Some commercial trucks stayed with the positive ground for some time afterwards, such as the 64 Freightliner I drove for a while.
Jerry


"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!