when/how should the rear fog lights be on
 

Hi,
The Check Lights Warning message is on and it is annoying to me especially I just replaced the whole set of tail lights to avoid this issue a few months back.
I also recently replaced the headlight bulb and everything was working up until like 2 weeks back. I checked everything is supposed to lit up when I turn on the lights except the rear fog lights. I don't recall when the rear fog lights should be on. Will they be on when I turn on the "normal" lights or will they be on only when I switch on the fog lights?
Please help. It is driving me crazy. Thx Tom

What model Mercedes are you asking about ? Thanks

oh sorry. Mine is 2000 E320 4-dr base sedan (W210). thanks

The "when" of the rear fog light generally is: Only in times of very low visibility/dense fog. The light is intended to provide an extra measure of visual range, that is greater than the tail lights themselves, for rear approaching traffic.
The word "The", above, is emphasized because only the rear fog light towards the center of the road is to be on. The left light in most of the world, the one on the right in Blighty and other countries afflicted with driving on the left.

Thanks Frank. I have replaced them with aftermarket ones, I am expected both of them to work. I know in North America, only the left one would work. The one on the right is a dummy one. In any case, I was wondering when they should get the power? I am guessing when I turn on the switch all the way to the right and then pull it out? I did that, the fog lights up front lit up but none in the back would do the same. I tested the power and I didn't get any power there. Checked the fuse and it was good also :(

9:

Most MB cars for the US market are not equipped for a rear fog light. It is possible that the rear wire harness contains that wiring, but if the headlight switch does not have a two step fog light pull-out the rear fog circuit will not operate.

911euro, the Owner's Manual describes how the rear fog lights are activated.

Thank you Ferdman. I misplaced my owner manual and I have owned it for almost 20 years since new. Fortunately a friend of mine has one. I now know how to turn on the rear fog light. It is embarrassing to admit that I never turned on the rear fog light before even once, and I am the original owner. -:(

I got stopped by the police when thr rear fog was lit on my w210 (it was foggy)

He advised me to switch it off as it appears that the brake lamp is inoperational. (It was last week of the month so he maybe fishing for tickets but I got away with a verbal and warning only).