2007 T800 Kw tail light back feed

[Chamberpains]

Not the ecm for sure. It has nothing to do with lighting.

It may be the switch, but I'd lean more towards a ground that is bad or crossed\shorted wires. Check and clean all frame, battery and cab grounds, trace you wires up the frame rail and see if theyre pinched, corroded or melted together Also i had a similar problem a few times and it ended up the trailer electrical cord itself had gotten pinched or the sockets for the trailer electrical cord had corroded and fused together behind the plug. Check the back side of those sockets on the cab and the trailer.