I used a trailer relay isolation package from Electrical Connection. Please try web link below.
http://www.electricalconnection.com/wire-harnesses/hrns-trailer.htm
My trailer was wired for a 5 wire system.(brake and running lights are separate) I used a round 5 hole plug connector. Please see Auto Zone web site link below.
http://www.autozone.com/R,NONAPP1089/store,220/shopping/accessoryProductDetail.htm