Quote from RonD on Grounds:
Ground Straps
Just as a heads up, ALL the metal parts of a vehicle are painted BEFORE assembly, and alot of the panels have rubber washers/grommets when assembled, so they are NOT grounded just because they are bolted together
The engine and transmission sit on rubber mounts so the main ground cable on the engine doesn't give the frame or body any ground connection
Battery negative is the Ground of course, its larger cable should go to the engine because the starter motor and alternator use the largest AMP loads
There should also be 1 or 2 smaller ground wires on negative terminal, they go to the metal inner fender and metal rad support, they are for headlights, horn and engine fuse box relay grounds
The back of the head ground to firewall is important as that grounds all the cab electrics
There should be a ground strap under the cab to the frame, and a ground strap under the bed to the frame, both on drivers side frame rail
These pass the ground from the engine/firewall to the frame
You can add a ground strap from engine to frame at the motor mount or where ever you can find an easy place to do it
Engine moves of course, so strap needs to have slack
This will give a double ground path for the cab and tail lights
The ground on the bed is for safety, i.e. no static charge when filling up
A Ground is only as good as its connection, need bare metal, no rust or corrosion
Best practice is to sand area(bare metal), put on ground strap then paint over it as needed
The main ground is usually on starter motor bolt, and that area can get oily, so wipe it down as best you can, the bolt and the surfaces and the cable end, so it will have a good bare metal connection