Ok when I plug in my trailer board, indicators work ok unitl brakes are applied then just brake lights.
EARTH PROBLEM you all scream, but where. The trailer board all lights have a common earth (which appears good 0 ohms on all lamps) leading to a common earth on vehicle socket which goes to good chassis ground.
I cant see how turning on another light (i.e. Brake light) causes another light (indicator) to stop flashing, even if it was a bad earth.
Grrr 4 years at tech college studying electronics and a simple triler board makes my brain hurt.