I have a 1990 740 GL sedan, automatic. I've had it for a year and a half now, bringing it up to peak condition. My to-do list is down to an odd switch on the dash between the headlight and antenna switches. I understand that it is for the rear fog lights. I guess my first question is if this is true.

Whatever that switch controls, it isn't working. There is a bulb on each tail light cluster that tests good but does not illuminate in response to any switch. This bulb is at the upper inside corner of each cluster. The fuse for the rear fog lights is good, swapped it with known good, still no result. The lamp within the switch does not illuminate. However, the conductivity between some contacts on the switch changes as the switch is thrown.

I guess those were the easy fixes. I looked at DC coming to the switch, testing the contacts in the white modular plug that snaps in to the switch. I either got no voltage difference or I got 10 millivolts, not volts, between some of the contacts.

Is there a way for this Saturday afternoon light-duty tinkerer to activate this switch without ripping my brick apart? Thanks!

IF it's the same as the 200's then the White/blue trace should have 12V on it. then when you toggle the Switch you should have 12V on the White.
The White is what runs to your rear fog lights

As mentioned, I tapped into the Fog lights with my brake lights so I have double the illumination when I step on the brakes.
You do have to add Diodes so you can use the Fogs separate from the Brakes.
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'75 Jeep CJ5 345Hp ChevyPwrd, two motorcycles, '85 Pickup: The '89 Volvo is the newest vehicle I own. it wasn't Volvos safety , it was Longevity that sold me

oh it is true but it is just a light on the left rear. It is something like the benz parking light on the 300e. Same thing.
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92 240 2.3 L with a Borla! 104,000 Miles !!

no its is both rear top red lights that work on that switch. some people even wire them up as 2 more brake lights...easy to do and safer to have.

My '92 745 only has a rear fog light on the left. The right lens has blank plastic in place of the socket. The owner's manual confirms this arrangement.

I wish I had them on both sides, as I would definitely do the brake light conversion.
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'92 745 B230F

It is definitely true that they are rear fog lights. My question is are your wires in ok condition?

Are my wires good? Well, I suspect that the wires that carry DC to the switch, or something else electrically upstream of the switch, is bad. I'm not sure what to look at next, except for pulling the wiring harness out, which I hesitate to do.