lights with minds of their owns

Hey guys,

What seems to be a basic lighting install is driving me crazy. I have a 20 ft. car trailer and I can't seem to get the wiring right on it. I have run ALL new wiring and new lights from 1 end to the other, my first question is what exactly is a common ground  The reason I ask is because I have checked wiring at the plug side of the vehicle its a stock TrailBlazer, never had problems with this and with my test light everything checks out ok at the plug.

Brown - Right Turn

My stop lights are 2 wire self grounding standard trailer lights. I have 1 wire connected to green (running lights) and the other to the corresponding turn signal color (red / Brown).

I will explain this the best I can. With everything plugged in and a test light in my hand, at the back of the blazer, i turn the parking lights on. The green wire is hot, i disconnect my test light, go to the back of the trailer, ground my test light, test the green wire, the green wire is NOT hot, test the red wire (left turn signal)the RED wire is hot. I disconnect the test light again and go back to the truck and now the red wire is hot, and the green has no juice. WHAT THE HELL is going on!? Like i said all this wiring is NEW, its trailer cable so its 5 wires, with insulations wrapped in 1 giant cable. The white wire is grounded to the frame of the (all steel) trailer, and connects to the correct wire at the plug / truck. Im pulling my hair out over this, is this a joke?

Thanks in advance guys i have NO clue as to what is happening here, I have wired TONS of things more complex than a stupid trailer and I feel like a moron!