Originally posted by Herritt
I think there there was something weird at that time with the voltage. It seems as if the brake/turn light was getting it power on the marker segment of the light unit. Its one of there sealed light units with three pin concector. Should there be any resistance between the marker wire and the brake/turn wire or should it read open?
Hmm, sounds suspiciously like a bad ground. Most likely, the big ground between your Airstream (A/S), and the tow vehicle (T/V).
Your dual-element bulbs, i.e. the combination stop/turn/tail light bulbs, share a common ground. If the ground between the T/V & A/S is bad, and you turn on the taillights, the power goes through the higher resistance tail light filament first. With no ground (return path), it then proceeds through the stop light filament.
Still in search of a ground, the power will back-track up the brake/turn signal wire until it finds the brakelights in the T/V. It will then pass through that filament, and finally find ground.
Since the brake/turn signal filament has less resistance than the tail light filament, you will probably see it glow even though you only turned on the tail lights.
But with so many resistances in the way, everything will be dim.
While I can't express things as well as my hero, markdoane, please feel to ask questions. Although, to me, it is fun to understand exactly what is wrong, you may find it easier to just thoroughly examine your ground connections. Start with the 7-pin connector - don't depend on the ground being established by the hitch.
Hope this helps,