It sounds like a floating ground. Check, clean, service ALL of the vehicle grounds. Disassemble them, burnish the connections to shiny bare metal and reassemble with a coat of antioxidant paste. Verify and service the following ground inventory: battery-to-engine, battery-to-radiator support, battery-to-right frame rail, rear of right cylinder head-to-firewall sheetmetal, instrument panel wiring-to-kick panel under the dash next to the emergency brake pedal assembly, tail/brake lamps-to-bed, front turn/park lights-to-radiator support, and headlamps-to-radiator support. In addition, check the tail & turn lamp sockets for corrosion and clean or replace as needed.