A very nice alternate to the wire splice is to take a dud PGM unit, chisel the plug off it, and have a good auto sparky fix it to the cut end of your HRC unit.
This way the HRC unit plugs into the PGM, and the loom plugs into it.
Not the cheapest way to go by any means, but this way it is easily removed and replaced.
An alternate to that is to have a sparky fit a common multi-point plug to each end of the HRC unit, and another set to form the break in the loom.

I dont wish to cast doubt on peoples ability to perform grafts themselves, but do suggest the aid of a good sparky, as I have seen more than my fair share of sub-standard cutting, soldering and insulating.
Given how much rides on the need for good electrics, this is one area not for the ham-fisted.