You dont need the harness. [unless you want to run hi and low beams at the same time which will probably burn your housings, if not wires]

When you install, unplug the 3 prong from the stock headlamps. Take the projectors and plug it in place of the stock ones. Turn on lights and check it. Thats it.

I went through 2 sets of projectors. I think I have creditbility