Could also be that the contacts and/or socket are dirty or corroded. Had the same thing with the rear indicator on the 280, flashed fast as if burned out, but was working. Careful cleaning of the contacts and socket cured the problem.
'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.