On my 1990 300te the defective lamp warning light lights up when I turn on the parking lights and stay on also with the normal lights on.

I have checked and replaced both tail lights and also checked all the rest.

But it has a pattern, when the RPM is low (at a stop light), the warning light will turn off, and then come on again when I start driving again. This is not caused by the brake lights.

Also, during these warm summer days it might be off for long periods while driving, it seems it is more likely to be off when the AC is on (or anything else that draws power). If it is lit on a stop light, working an electrical window usually turns it off.

Does this seem like an earth problem? Any ideas?

Thanks! jm