Mercedes Missing Wire?

[Bryan Endres]

I have a friend with a 2003 Mercedes (I forget the model) who noticed that one of his tail lights was out. In his case, the inner tail light in the right lamp assembly. He changed the bulb and the light is still out. I tested it with a multimeter and found that the bulb is good but there is no power on circuit itself. The assembly is a single panel with simple circuitry on it so it is easy to map which pins are for which bulb, and therefore which wires are for which bulb on the connector. I went to check the wiring on the connector and found that there appears to be a wire missing. I checked the other assembly for reference and found that it had six wires while this one only had five. Each assembly should (theoretically) have six wires – five bulbs and one ground. I tested the left assembly and found that there were indeed two hots when the lights were on, one for the outer light and one for the inner (you can cycle between one or both by turning the fog lights on / off). I looked down behind the trim and there were no wires hanging as if they’d been disconnected but there was nothing to be seen. The harness looks factory and isn’t loose as if missing an entire wire so I’m quite perplexed by this. Has anybody heard of a car with a wire missing apparently from the factory?

A couple of clarification notes: He has only had this car for a short while so the history is unknown other than a minor accident several years ago in the front of the vehicle. It is also equipped with a system that will warn the driver if a light is burnt out. We triggered this when we unplugged the whole module, but the system reset and didn’t report any faults after putting it back together, even though the one light was still clearly not working. Is this perhaps some strange design by Mercedes? He still has working tail lights and brake lights, it’s just that this one inner one on the right side doesn’t light and doesn’t appear to have any sort of power connected to it.

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