You could ut a red led in one of the BU sockets and wire such that it lights with the rear fog switch and in reverse. Getting the bulb integrity system to work might be a real problem.

On my cars that do not have a rear fog light I am cautious as to not hit the rev limiter during inclement weather. The BUs are just not bright enough.

 

When my parents got stationed at RAF Mildenhall we had an American spec KIA Rio and we had out lights reprogrammed so we had one reverse light and one rear fog light. Only cost $240 to convert the headlights and the rear lights.

 

Mildenhall, that was the base in Suffolk I went to as a kid to watch jets take off. In East Anglia there were a lot of bases like that so I may be mistaken. I once saw a group of Sabre jets fly over our heads as we slowly passed, a big thrill for a kid. Bombers were the usual plane seen.