schematics are around online. However, I for one have never seen one that is correct in all aspects. The only people with lengths and that kind of information are the people who build repop harnesses (I would think).
Is there enough of the old harness(es) to get measurement off of?
Then write down what you want to change, like adding turn signals.
I use a piece of weed whip string. Get a length for each run from that using a sharpie to mark the string. use a tape measure to get real numbers off of the weed whip string. The weed whip string works so well because it doesn't pull tight but has a bend that is just like the wire.
Note: I worked for a company that did many, many cables for the (now cancelled) Marine Corp Expeditionary Fighting vehicle. This is how we got the most accurate measurements to make the cable assemblies for that program. General Dynamics had the best modeling software available, but it never figured cable lengths right, there was always some change that affected basically everything. They laughed the first time they saw me do this the first time. The first time they saw finished cables, they increased the scope of our work for more and more.
You don't have to do this step, but I grab a piece of 4X8 plywood out of my shop and start laying the harness out on it with nails at each breakout, end etc using the measurements gotten from above. Grab your roll of #14 wire and start laying in the runs. Leave extra wire at each destination (6 inches or so). I start at the light switch. The most critical breakouts, position wise are the the ones right near the light switch: the ignition switch wire, the wires to the instrument panel (leave these longer than original, you will thank yourself), and the circuit breakers. Then you are at the "T" where the harness splits and goes left and right. That "T" wants to be right up against the firewall. Im sure you can figure this out. LABEL BOTH ENDS OF EACH WIRE AS YOU APPLY IT TO THE BOARD.
It is a bit tedious, but the results are worth it. When you get the wires done you can either finish wrap them with non adhesive tape or just temp wrap them with sticky electrical tape every 12-18". Then fit check the whole thing into the vehicle. If it is your first try I recommend the temp wrap.
Then its time to put each wire exactly where it goes and cut the excess off. Remember your service loops. A rule of thumb for harnessing is that there is enough extra length on each wire for two re-terminations. Its not much, 1/2-3/4" extra.
Add connectors, and markers if you have them, and do the final wrap. I start at the farthest away with one roll. As I get near a junction I use a second roll to wrap that leg from the far end, then over the joined bundle for a few inches and cut that new tape. Then continue wrapping the first over all that and continue adding junctions you come to them. When you are done that first tape should be continuos from the farthest point all the way to the light switch. That is the only place a few turns of adhesive tape are used.
I've done lots and lots of vehicles this way. I even used to offer the service. But all the harnesses on an M37 take 100 hours or more to do RIGHT! You can quickly appreciate why the repops are so expensive, they are labor intensive. I'm particular though. I have yet to see a repop I would use on one my projects. But then again an original isn't perfect either. The ones I spend my time on are suitable for a Concour de"Arrogance, LOL.
Give it a try! Its just time and money, but less money than a repop. New original connectors don't come cheap, unfortunately. You can use old ones (but not rusty ones) as I described above. But you still have to hunt down the rubber boots. DO NOT USE OLD ONES. Skip the polishing of the shells before plating and the clear coating after plating, thats for some silly old man (like me).
Good Luck!
