As the chassis, the engine and the transmission are connected to the battery negative terminal, most systems will only receive a positive feed. The current returns to the battery through the chassis. This means that the component mounting and the body form part of the circuit.
For example, lights may shine dimly as another component is operating which shares the same earth. Wiper motors, radiator fans, and other motors may run slowly, and the operation of one circuit may have an apparently unrelated effect on another.
On many cars, earth straps are used to connect parts of the cars (engine, transmission, etc.) to the body, usually where there is no metal-to-metal contact between components due to flexible rubber mountings
To troubleshoot earth faults, disconnect the battery and use a ohmmeter (voltmeter set on resistance reading) to check the resistance between a good known earth point and the earth connection being tested. The resistance should be 0 (or close to 0).
If high resistance is read, dismantle the connection and clean back to bare metal on both the bodyshell and the wire terminal or the component earth connection mating surface. Remove all traces of dirt, corrosion and paint to ensure a large metal-to-metal surface connection.
On reassembly tighten the connection securely. If terminals are used and the connection is exposed to the weather, then apply a suitable grease or sealant to protect the connection.