I had the following electrical mystery occur last week.

86 300SDL, 241,000 miles.

As I'm driving back from the job site, I notice that the overhead courtesy lamp and the underdoor lamps are on. Well, glowing, not completely on. The courtesy switch is in Auto.

I discover that when I open my door or the passenger door the lamps go off, and they come back on dim when I close the door! now that's just not right!

Playing with the courtesy lamp switch reveals:

Auto - lamps are dim, go off when doors are opened.
Reading - light comes on full bright, door lamps go off.
Off - turns all lamps off.
Courtesy and Reading - lamps come on and the doors go off.

Switching between settings operates the lamps, and curiously, the Fasten Seat Belt warning in the overhead energizes and flashes, as if a door had been opened, but without opening a door.

I give up, and pull up to my favorite Chinese restaurant for dinner.

In the parking lot, I open the doors, remove the switches, find nothing amiss that I can see. No apparent bare wires, broken wires, etc.

Then I wonder if someone messed up the wiring in the front passenger door when the power window motor was replaced. I discover that one of the hooks on the door panel hasn't been engaged, but no wiring problem that I can find.

I discover that the lamps go off when I turn off the key. Good, I don't ahve to worry about running the battery down overnight.

Then I find that the driver's lock operates but no others, since apparently the central locking system has now quit. More bummer!!!

I call my wife for sympathy, since I spend several days a week away from home in the line of work. Dinner was consumed in a bit of a foul mood, as might be expected, and didn't quite taste as good.

During dinner I wonder how these could be related. I decide to check the fuses, figuring that a blown or loose fuse could easily explain the non-functional locks. After dinner I open the fuse box lid, look and see no blown elements. For grins, and because I can, I touch and rotate each and every fuse one in its holder. Nothing wrong that I can see.

Son Of A Gun! The central locking is restored, and the courtesy and door light function returns to normal. I'm extremely happy, relieved, and puzzled at the same time.

So, I'm now wondering how the apparent loss of power to the central lock motor caused a backfeed, or a sneak ground, on the lamps. I guess I'll have to research the wiring diagrams.

Oh by the way, my fortune cookie said "Your tide of fortune will be turning" and it did, so I now have to start believing in fortune cookies, too! :p

I love a good puzzle, and I hope you all do too!

Best regards,
Jim

P.S. I'm an electrical engineer by profession, which makes it even more interesting...