Regarding harnesses for other Ecotec projects, I don't believe DF has ever sold standalone harnesses (reworked or not). The harness service has always been, you box up your donor harnesses and ship to them, pay them, and they rework/combine the harnesses (specifically for the Goblin, not other Ecotec projects) and ship it back to you. This service has always accompanied a Goblin purchase, but I am sure they would do it for you, even if you're not the original buyer of your kit.
The wire extension kit you purchased is basically to enable you to rework your own harness exactly as it's done in the videos, with the Goblin in mind ... meaning, the wire extensions are sized and colored to match exactly what is needed for your finished harness to fit in the Goblin. It was never designed to be a kit for other Ecotec projects. It's also not even truly necessary, as there is plenty of wire stripped out of the harness that can be cut to size and repurposed.