The FF MIGI Harness was all black as I recall but the wires were marked as to their function. I allways thought the harness was decent and so were the diagrams. Also the harness was included with the kit.
After FF was acquired by CMC the FF VW TD and CMC kits were identical, save for a couple of emblems. The wiring harness was optional but color coded, but the colors changed over time.
If you are rewiring an early FF MIGI with an all black harness you will need to trace and mark all wires. MCM electronics has kits for marking wires. On the colored CMC harness you probably will have to run a continuity test on all wires anyway just to be certain. Your other option is to scrap the harness and go with a custom made unit attached to a GM fuse block that comes with colored wires that are also marked for function.
Thats what I did on my last VW project TD and while the harness was very pricey i consider it a good investment for the time I saved and the peace of mind. All terminal,resistors,loom etc were included so I did zero running around.