!!! Passanger Back Running Light Wont Stay On !!!

[mater]

HI I HAVE A 1971 FORD F100 AND MY BACK PASSANGER TAIL LIGHT WONT STAY ON THE BRAKE LIGHT COMES ON BLINKERS WORK FINE. I REPLACED THE WIRE HARNESS TODAY AND BACK TAIL LIGHTS BULBS TO MY SURPRISE THE RUNNING LIGHT CAME BUT THEN WHEN I TRIED IT AGAIN RUNNING LIGHT NEVER TURNED ON AGAIN . PLEAS E HELP IF YOU HAVE ANY SUGGESTIONS. THANKS!!!

 

[BKW]

There are no running lights. The taillamp bulbs (#1157) have a double filament.

One filament is used for the taillamps, which only come on when the headlamps are on, the other filament is used for the turn signals, brake lamps.