Test the ground
In a car your age, the first thing to check is a good earth ground.
Set your digital multimeter to DC Volts.
Test the voltage at the battery with the key on, engine off. Multimeter + to + (positive- red) onn the battery . Multimeter - to - (negative - black) on the battery. Note the voltage
Connect the + lead to the positive terminal of the battery.
Connect the - lead directly to the earth wire or ground contact at the rear indicator socket.
Turn on the indicator or keep the system active, ensuring contact with bare metal.
Read the display: A reading close to your total battery voltage (e.g., 12V) indicates a healthy ground with no major resistance. A reading close to 0V or dropping significantly means a bad ground connection.
If it is a bad ground, start tracing how the battery grounds to the car, and where the indicators are grounded and then work toward the middle. With a car of this age, corrosion is usually the culprit. It can be anywhere but connectors and connection points are the most common problem. If you find it is a ground and you just want to get it working, you can always buy a black wire, connect it directly to the indicator ground and run it back either to the battery or a ground along the way that shows full voltage.
Alternatively subject your 411 to a major electrical refresh. Pull all the fuses and polish them and the slots. Examine all the connectors, clean them and apply dielectric grease. Remove all the fittings that have a ground wire and give them a good refurbishment. If you find any bad wires, bad insulation or bad connectors, replace them.
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