If your bike has that 4 pin male connector on it, it is absolutely the wrong gender, vehicle side should have female sockets for the powered circuits and a male pin for the ground, that connector needs to go.
If the PO had a trailer that needed a 4 pin harness then there must be a 5 wire to 4 wire converter somewhere, hopefully it is also an isolation unit.
If you want to stay with the 4 wire set up, you need to change the male connector to a female and rewire the trailer using a 4 pin male and set up the brake and turn signal lights to "share" so they act as both brake and turn signals.
If you want to stay with the 6 pin connector on the trailer you will need to install a 5 way isolation trailer harness and figure out which pin in the 6 way on the trailer does what.
The link that Andy pointed you to is for bikes from 2006 to 2017, I am trying to get additional information on earlier 1800s to see if my system will also work on them, but I need a "donor" bike or someone willing to do some digging around.
Mountain rider has agreed to do that, just need so input from him.
Keep us posted on how you plan to go forward.