Tail lights, turn signals, and brake lights are not working on my 87 chevy caprice classic. Bulbs and fuses are good. Thinking it may be the ground wire but need to know where that wire is located...

Look Under the Trunk Matting At Driver side Look For the MAIN GROUND on the Rear Lights Harness, The GROUND WIRE IS EITHER Broken or Has a VERY POOR Connection , You May need to Replace the Connector ITSELF and NO Matter What you Find RELOCATE THE GROUND CoNNECTER HOLD DOWN BOLT to a Spot that YOU Preparer With GRINDING the Mating Surface where the Bole & GROUND WIRES Will Mount. Also Do NOT RULE OUT the JUNCTION PLUG In the SAME AREA Of the Harness Be DURE of >>CLEAN<< I am About 99% Sure of these Being your Problem,Given The Fact that I Have Seen this Literally Hundreds of times
Good Luck With it !

87 chevy caprice no running head and dome lights are not working and also the door buzzer when the key is in the fueses are good