That's pretty standard. Mine is the same way. I also have a 5 wire set up, into a 4 way plug, but no brakes on my trailer.
You have a wire shorting out somewhere, and my bet is its one of those wires to the clearance lights. You can't find it unless you pull all the wires through the frame, so its just as easy to replace everything if your pulling it all out anyways.
S.