I cannot agree with javalion, when two or more items share a ground, just because one item works, you cannot assume that the other will. For instance, the ground that will light an interior light will NOT run your starter.
What can happen with bad grounds is that the bulb IS working, but it is not bright enough for you to see! Or the item completes a circuit through another item, this is what causes some of the strange symptoms you see with a bad ground.
I also have little faith in just taking voltage readings. You need CURRENT to do work.
When the right side headlight did not work on our '88 745 GLE, I tried a new bulb - no good. Then I measured the socket and got 12 volts like I should. I suspected the plug and soldered in a new one - no good, I still had my 12 volts. I measured the current, there was some there, but only milliamps.
I stripped the wires back, and about 2 feet from the headlight, there was only one strand left, among a bunch of black gunk.
I had to go to the junk yard and replace the wires all the way to the firewall to get the headlight to work.
With our '90 744 TI, the car would stall when I put on the high beams. It's in the FAQ! Bad ground in the left front fender. Enough ground for the ignition system, but the high beams stole it away and the car stalled.
BTW, one quick way to check a ground (or to repair one in some cases) is to add a temporary one, like a clip lead, or even a jumper cable.
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'96 855R,'64 PV544 driver, '67 P1800 basket case, '95 855, '95 854, the first three are mine, heh, heh, 435,000 miles put on 9 bricks
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