There used to be a sticky thread in this forum on just that subject with solutions for an isolated trailer connection. Found it,
it was moved to articles HERE. I have electrical diagrams if you are interested for a DIY one which is what I used on my Gold Wing and ST1100.
Electrical Connection has a universal kit HERE but it is a little pricey. Mine is also what I consider a universal since I've used it on many other bikes besides my own (different makes/models) and is not specific to any. My current version of it is in the photo from my Flickr albums. You would just need to pay attention to the upper part of the diagram unless you also want to add a LED light bar with brake and running tail light to the rear of the bike (which I did on my CTX1300). This diagram is what I was intending to use for my current bike and the same as for my past bikes. But then I decided not to put the trailer part on and not pull a trailer... yet. I can still add that part later with what I already have installed (I did add the LED bar). The wires used to tap into for left, right, and brake relays may likely be different color codes for the ST1300 than for my CTX if Honda is not consistent there.
You can click on the photo for a larger view and also to go to my Flickr album to see other diagrams. In my gallery in this forum (panel on the left) I have photos showing detail of how I wired this on my ST.
Oh, I only show a 5 wire trailer plan since the bike is native to providing a 5 wire setup. Most made for motorcycle trailers do have a 5 wire setup as well... or should. The 4 wire systems that still hang around in the USA are OLD old school that were common due to American cages only coming with 4 wire setups. All vehicles overseas (both east and west of the USA) are 5 wire internally so makes sense to stay that way. That only means the turn lights are separate from the brake lights for a 5 wire setup. More lights in back = better.