Driving on the motorway at night recently, I have come across a few motorists with one back light not working. Assuming they don't know about it, I have tried to let them know. Once I have overtaken them and hit the hazard lights but I think they thought I was thanking them for something! Another time when I passed another vehicle, I turned on and off the rear fog lights a couple of times, then indicated on the side of the missing light. Is there a night-time signal (safe of course) that could alert a motorist to this?