I was in the same situation with my BN-2. What I did was to sit in the living room with the new harness ( and the old oily one in a box nearby), the wiring diagram and color code key (enlarged to make it easier to read), and a label maker (the kind that prints out small labels that can be self stuck around the wire ends) to label all the terminals and junctions. Once you have the elecrtical components installed in the car and the harness all labeled it is very much easier to lay the harness out next to the car, and then to see how you must route it in the car. It is most certainly a bit of a puzzle, but it worked for me. The various restoration books and the photo albums that they sell at British Car Specialists in Stockton, CA were a great help. I had every photo in the Austin Healey club magazines I have post-it tabed if they so much as showed a wire. A good place to start is with the instrument panel area because the lighting terminal ends are obvious. The harness is also thickest there and there were a few moments when I was sure it would be impossible to connect some wires to various switches or gages. I'll bet the guys who fit the doors, the seat cushion covers and the wiring harnesses were a cranky lot until they got a few under their belts. I probably spent 8 hours labeling, and another 15 hours putting the harness in the car. Do not skinmp on clamps and do not leave wiring in positions that cause chaffing of the wire. If you have the old harness, do not throw it away until you are done with the new one. The latter, I find, tends to be a good idea for any parts that you are not going to reuse. I was happy to go through the cut out frame parts I repaired to see just where screw holes were.
Jon