Problem with right brake and indicator lights?

I have a trailer tent (sunncamp 350) and there is a problem with the right brake and indicator lights.

Indicator is fine, until brake (or headlight) come on, then indicator becomes very weak and barely visible... and brake light flashes.

When the brake light is not on it does flash alternately with indicator very faintly (only visible with cover off).

Its not the towbar electrics as I've checked it on another car - same problem.

Have also replaced all bulbs - no joy.

Can't find any fuses on trailer.

Any ideas?

Thanks


Will