This is a more expensive route but simple.

http://www.foreignautosupply.com/parts-accessories/74-new-taillight-harness/#.VaLDLyPD_qA
 On Jul 11, 2015 1:26 PM, "Spencer Allen" <spencervw@aol.com> wrote:

> Hi all. I want to wire up a flat 4 plug pig tail for towing a trailer on
> my 85 Westy. Should I use a powered converter or just use a non powered
> plug and splice in to the tail/brake turn wires? If I use the powered plug
> where will the heavy red wire splice to? Anyone have a diagram for hooking
> up a trailer plug in an 85 ? I probably can figure it out but don't want to
> butcher the wiring. Any help is appreciated. Thanks.
> Spencer
> 85 Westy
>