Bought a used trailer from a friend and it has a 6 wire plug on it. The harness that came on my Road King is a hack job 5 wire that was spliced into the brake light harness. Are there any harness that can be bought for trailers or do they have to be made a spliced?

 

Sounds like you are looking for a plug-n-play wiring harness. I recommend you go with an isolator to protect your motorcycles electronics. A number of different companies make them. Below is an example of one.

https://www.namzcustomcycleproducts....ler-isolators/

 

Yes buy an isolator and wire it correctly.

Bought mine from...http://www.openroadoutfitters.com/mo...trailer-wiring