I've an 85 K5, my left hand turn signal is playing up. All lights work as normal, the ones in the grille, sidemarker etc. But when I turn the turn signal lever as if to turn left, the front left(grille) and side marker lamps don't flash, neither does the lamp on the dash, but, the rear left works as normal. The front and side marker lamps are lit, as normal, just don't flash. Ideas anybody?