Another possibility is corrosion - in our '88 745 GLE we had a headlight that would not light, but I measured 12 volts at the plug.

This did not make sense, so I began stripping the wire back from the plug - one foot back along the wire I found a pocket of black gunk and only 1 strand of copper wire. One strand was enough to show voltage, but not enough to light the bulb.

I replaced the wiring with wires from a junkyard back to the firewall.

The idea of making a loop sounds good, or, you may have to break into the insulation every few feet to find the problem.

Adding a new feed at the rear of your car might be a reasonable temporary (or permanent) solution.

Dear Simo,

Hope you're well. I suspect a broken wire. If a wire is flexed excessively - or pinched - the copper conductor can break, while the insulation sheath remains unbroken. These can be hard to find.

One way - if the wire is accessible, is to form a narrow loop, which is then moved along the wire. If - at some point - the rounded top of the loop suddenly gets pointy, the break has been found. To apply this search method, one grips the wire between thumbs and forefingers, and feeds the wire from one side to the other.

Another way is to back-trace the wire. That requires a wiring diagram, which I do not have for any 1992 model. I presume your car is a wagon (estate) model.

Hope this helps.

Yours faithfully,

Spook