Digital multimeter.

If you suspect a ground loop in your 12V DC installation, turn off the plug pack and put a voltmeter between the positive battery terminal and the disconnected positive cable.

You need to ensure the COM (negative) terminal is to the disconnected positive cable and the V/Ohms/A terminal is to the positive battery terminal in order to get this little test right.

If your DMM now reads zero, there’s no electrical ground path in your installation. If there’s a voltage then you have a ground fault. Typical causes of ground faults are moisture or drips, torn insulation on moving wires, and poor terminations and busted switches.

If you find a ground fault, you can work out how bad it is by switching to ohmmeter and measuring between the disconnected positive cable and the negative battery terminal. If the reading is 10,000 Ohms the fault is probably moisture or a poor termination. If you find less than 10 Ohms, there’s a major ground problem.