Another place to look is above the driver side rear axle where the exhaust pipe passes close to the rear lamp harness anchored between the flanges of the left frame rail. Exhaust heat can compromise the PVC wire insulation resulting in cross-feeding, short circuits and grounds. Melting of the harness becomes acute if the plastic anchors retaining the harness to the inside of the frame rail are broken, allowing the harness to sag out of position. In cases where steel harness clamps were used in lieu of plastic, wire insulation melted by exhaust heat can allow rear lamp wires to ground to the frame as they pass through the clamps.