Before you begin a restoration on a Mustang, one note of advice.......DO THE WIRING LAST!!!!! I have encountered numerous situations where a Mustang I was working on had just come to me from a body shop and they really screwed up the wiring. The harnesses were painted over, unwrapped, spliced into, and even mis-wired. Even some Mustang restoration shops just don't seem to treat the wiring of the car with any kind of respect or importance. A wire is just a wire. Well I must disagree. The wiring system of your Mustang is it's nervous system. Would you like a brain surgeon to be in there and totally disregard the nerves to get to a tumor only to have an eye or a leg not work right anymore? Sometimes when a body shop welds panels in or installs interior pieces, the wiring is mishandled, crimped from a panel installed, or outright cobbled. You may find that after you get the car back, some things won't work right or fuses will keep blowing from a direct short somewhere. Also there are many aftermarket wiring kits that you can purchase to rewire your Mustang. I'm sure they are great products but there are some issues with some of them, such as having to cut different holes in the firewall or not having correct connectors. My personal opinion is to use factory correct reproduction wiring harnesses whenever possible. Everything is made to work together and plug in like it should.
This is what I found when I removed the cluster of a 1966 Mustang Coupe I was working on that had been purchased by the owner 6 weeks earlier. To put it mildly, the owner had some electrical issues. The harness wasn't even rewrapped after it had been worked on and thrown back into the car. I won't even mention everything that was wrong with this situation.
 
I can't stress enough the importance your wiring system has on the performance of your Mustang.
Treat it with the same respect you would with the engine being built correctly or the brakes working properly. You will have a much more satisfying and safe Mustang to enjoy for years to come.