The same harness the kit came with, just a brand new one so I can get away from having so many spliced up wires. I'm a perfectionist, it wasn't necessary but it was only about $20 to buy the new harness so I couldn't resist.
The Pioneer head units are known for blowing their internal microfuses, and mine just happened to blow after the door kit's wiring shared the same battery wire, probably causing the RCA wires that are normally away from all power wires to now intersect the door kit's power wire. The most frustrating part has been searching for the bad ground, since I didn't have any problems at all before installing the door kit.