Working on the dash. Wiring nightmare! Cut wires and open connectors.
Time to do some research and hit the books.
You may already be aware of this, but if I don't mention it I will feel I have not provided help to someone who may not know. My GoTo site for excellent schematics for First Generation Mustangs, especially beginning in 1972, and more especially with the 1973 manual, is Forel Publications.. And their prices are very reasonable. The 1971 and earlier schematics are useful, but not nearly as detailed as the 72 and especially 73, schematic files.
There are a few differences between 71, 72, and 73 schematics, to include:
- In 72 and 73 there is Circuit #640 that powers a Throttle Position Solenoid (TPS) that is on mounted some (not all) engine carburetors, Red Wire with a Yellow Hash tracer. That TPS circuit is not present in 1971 wiring harnesses.
- In 1973 the electric choke, powered from the Alternator Stator terminal (Circuit 4, White with a Black Stripe tracer) was introduced, which is not present on 71 or 72 model harnesses.
- There are be some emission control components in 1973 not on the earlier year Mustangs, related to vacuum control switches for components such as the EGR valve in some engine calibrations.
In any case, I recommend getting the 1973 schematic, in addition to the 1971 schematic for your 1971 Mustang. Both are useful, the 1973 schematic especially.