DIY adapter harness, needing insight.

So i'm trying to determine how adapter harnesses are made.

The starting piece is an ECU PCB Header: Attachment 1

the end result is something along the lines of rywires adapter patch harness shown as attachment two.

what's the process under the heatshrink? are they soldered? any prep work?

On the ECU header, it is wires soldered onto the pins, with adhesive heat shrink over the top of each for strain relief, and with the splices, they are crimped with adhesive heatshrink over the top.

From my experience the wires/pins tend to break at the header. On some harnesses I have seen where they pot the pins by casting epoxy resin around them for better support. You might want to consider adding that step.

After you have it all done you can all ways fill in with non conductive epoxy to strengthen and better relieve any stress that may occure