Dash lights causing turn signal woes

[mrlobster]

In my '72 GMC I put in a completely new wiring harness from AAW- under dash, lights, and engine. New gauges, switches, etc. Also a new Ididit column.
The exterior turn signals work fine at all times.
On the dash, when the lights are off the turn signal indicators work as intended. When the lights are on and the dash lights are turned all the way off- everything is fine.
When the dash lights are turned up, all the lights including the turn signal bulbs are on, and then the turn signals flash opposite to the turning direction (signal left, right light blinks).
Ground issue? Short? Bad switch? All of the above?

Thanks!