midlife's post in Junction block fender apron - purpose? 1973 Mustang special wiring... was marked as the answer
David, I don't see the connector from the battery, so I can't tell if your wiring at the starter solenoid is reversed or not (it actually can work by swapping the starter and the battery connections).
It doesn't, technically. The convenience wire simply needs a hot connection and a circuit breaker to provide power. When you have a tach dash and the isolation post, there are two leads on the main headlight harness that are hot: one goes to the starter solenoid and the other to the isolation post where it is married to the alternator output line. Inside the headlight harness, the two main leads are tied together so either one can be tapped for use as a hot point.
If an isolation post was used with a standard dash and the Convenience group, then the isolation post was simply used with a jumper wire from the starter solenoid to de-clutter all the wires around the starter solenoid. I don't think Ford did this, as it would require two extra parts that are superfluous and the bean-counters would have a conniption!