Strange turn signal issue on 72 C-10

[72bowtiestepper]

Wrapping up the multi-year restoration build on my 72 stepside and have run into a problem with the turn signals. With headlights OFF, all signals front & rear including front marker lights & dash indicators work great.

With headlights ON all signals are REVERSED....Switch lever down for a left turn..the right signals front & rear are activated. Switch lever up for a right turn....the left signals front & rear are activated. The dash indicators are opposite also. The brake lights are fine and work with headlights off or on.

I searched in old forums about issues with turn signals & found many people have had grounding issues, so I ran a dedicated ground to each signal housing front & rear.....No change. With headlights on signals are still reversed left to right & right to left. Any ideas what could be causing this ??