Buy a new wire harness(about $20) then use a fish tape to pull the wires through. You might need to drill a bigger hole on the bottom at the rear of the trailer to feed the wires to the end of the fish line. Im assuming you only have rear lights and no side markers. although If you do, use the same method. I did mine with side markers. It wasn't that hard. If you dont have a fish tape you can always call your electrician friend. If you dont have one, figure about $60 at Home Depot.
As far as your original wiring... the yellow, green and brown go to the rear lights. The white is a ground and those red ones could be for the side markers. Im not positive on that without seeing. I would use a tester to find that out.
heres a fish tape.
http://thumbs.ebaystatic.com/pict/7588821037_2.jpg