Well that is very interesting. First off I noticed you mentioned the fog bulb used is the same 21W bulb the brake light uses? Basically you are saying its like having something as bright as your brakes for a running light in conditions that are harder to see than normal. I take it that a normal fog lamp bulb such as the H3 that is used in front fog lamps would be too hot for the chamber and possibly melt the tail lamp?
The socket behind the empty chamber does not have a spring plate behind it like the rest of the bulbs since it is not in use. How exactly could it be changed so to accomadate a bulb?
Now I don't think that it would be possible to hook up a system that would function off of the factory light switch. I know that there is a second stop after the fog lamps but nothing happens. I have absolutely no idea what any of the wires would go to or come from. You mentioned that there is a amber bulb that lights up when you pull it to the second stop. Is this the little round thing in the middle of the knob? Mine does not light up since I do not have that system.
I think the most I can think about doing is do a simple splice job and get another 21W bulb in the empty chambers to work in unison with the original brake lights. I believe if it is just simply spliced so that the power can run in and back out the dummy light should not go on because the power is still being returned. Thanks for the info, and anyone else who knows about it or has done the conversion please speak up!
Robbie

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Rob C
89 190E 2.6
Beige