Greetings. I, too am dealing with wiring in a Fiberfab MIGI. I don’t believe I’ll need to replace the whole harness, but I do have a couple leads on sources for you. Custom Wiring (http://www.customwiring.com/hotrods.html) says they have a harness for VW MGTD’s. Another forum member also suggested Painless Wiring, Centech and Ron Francis.
Best of luck.