there is also more to wire harnesses depending on year you need to match the options your car had. a 71 convertible harness would not work well in a 71 mach 1 for the engine bay. behind the dash if you had a A/C car there are differences from a standard heater car.
It would be difficult to find a replacement for exactly your car, so the best bet is picking up a donor harness then using that to make repairs to your existing harness or sending it to mildlife for refurbishing.
I used a 1973 Convertible as a donor for my early mach 1 1972 A/C harness. the end connectors are the same, things like sockets for lights and sensors for brake pressure. The wires them selves are different colors but have the same function, a connection wire in 1972 might be black with pink marking, and on the 73 might be yellow with a red marking.
functionally the connector might be the same but stick out like a sore thumb if somebody later needs to make electrical repairs.
some sockets are very different but can be modified to work on the early cars. example a 1973 sport lights grill used a 3 pin connector because the sport grille worked as turn signals and running lights. on 1971 1972 cars there was separate lower turn signal lights and the sport grille lights were running lights when the headlights were turned on. it is possible to rip out the 3rd lead from the 1973 lights and rebuild them to work with the 1971,2 cars. you have to at least make sure if you have a gauges car that you do not use a donor from an idiot lights car because the connections will be different.
the biggest change from 71 to 72 was in 71 the convenience wiring group was integrated into the main harness, this made dealer installs easy for accessories that a car was not equipped with from the factory. for 1972 ford took the convenience wiring out of the main harness and created a daughter wire harness that plugged into the main harness in a few places to allow accessories to be installed. But there was a Cut of point for this change in August 1971 for 72 model year. so early 72s have the integrated 71 harness. Basically it makes things a bigger pain when searching for a harness. also even early 72s had a small change to the harness in removing the wiring for the map light. for a 71 if you want to add a map light you just plug it in. for 72 even early 72 the map light connection was on a daughter harness. so unless you hack into the fuse box or find the map light harness you have no way of hooking up the maplight OEM style. here is the kicker,,, they took out the maplight connector but left the footwell lights connectors which tied into the maplight. no wonder ford was losing money with all the different wire harnesses.
The best is saving your original harness and then repairing or restoring it. sometimes this is not possible due to rodent or fire damage but it is the best bet.
rebuilding the harness is time consuming after unwrapping the wiring you will find the wires are molded into cast plastic/rubber holders and the wiring clips are a separate strip held on the wires by the covering wrap.
I ended up cutting the plastic molded pieces which allowed me to replace entire strands of wire back to a problem area or allowed me to splice in accessories i needed. after the wire was soldered and heat shrink protected i would lay the wire back through the plastic/rubber holders and reglue them shut later i would come back with the clip strip and recover with black wire wrap. took me about 2 days of work to rebuild my harness. I even went as far and opening up the molded connectors and running new correct color marked wire for a few runs. you always find, damage broken wires or areas where somebody hacked the harness to install something.
