[whynotthinkwhynot]

Add some grounds. One that might be missing is a ground to the cylinder head. Yes, the big cable is connected to the transmission and the engine, but the intake manifold- where all these parts are likely grounded via the harness- is only connected by the intake manifold bolts. Typically there is a supplemental ground on the cylinder head. Find a screw or bolt somewhere that is holding down a bracket, and run a wire from there to the chassis.

Your problem could be on the hot side also, so you might want to investigate the harness that all those wires go into. IIRC, those should all spring from one harness. Look for any wiring damage.