RepairPal Certified shops can diagnose and repair your 1996 GMC C1500. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.
You have to get down with a voltmeter and a test light and trace your power through the wiring. Start with the power at the fuse, through the switch, trace it through to the bulb sockets. Do the same with the ground circuits until you find it.