Take a closer look at the back-up light bulbs. Might have one that is fixing to completely burn out. I always give the bulb a quick flick to watch the filament, ones on the way out will usually break at that point.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.