So I coded the rear fog lamps with Carly to illuminate with the tail lights but only the left side lights up - 2009 30i E70. I pulled the bulb, lightly sanded the socket contacts (have done this before!) and also the contacts in the light base itself. Also replaced the bulb. Still no light. Usually enough cleaning of the contacts will do it. When I touched a 12V test light on the metal base the bulb socket engages - no juice.

Is there anything else to check here? It's too bad, the other side is fine. I did note some rust on the riveted assembly, perhaps corrosion that is harder to eliminate has spoiled my fun.