... can’t find the wiring harnes....
Well then, its kinda like this. If the trailer hitch that you have is the BRP OEM hitch, the wiring kit / module came with it. If it was a model year 2011 or 2012 hitch kit, the trailer plug is a 9 pin round plug mounted on your rear fender. Not all 9 pins were used, however. If the hitch kit was a model year 2013 or later, the module also came with it, but in this case, the trailer plug is a little square 4 pin plug that hangs down under your left side saddle bag compartment. The trailer harness that you might have depends on what year the hitch kit was purchased and installed.
Of course, if you bought just the hitch used, then that's all you have....no module kit.
If you're sure that there is no OEM module / plug installed, then the most cost effective alternative is the Show Chrome harness. It's also an isolated harness and will terminate with 5 wires. You supply your own connector depending on what your trailer will have. Seems that the typical non-BRP trailers are 5 wire.