Unless a noticeable damage, normally a good mantenience is quite enough on the dash harness. Even with some damaged spots, those are easy to reach and fix And there is not NASA science on them. Usually ugly splices made on the past and sometimes even with some wire melted ( such as the alt wire ) it could be easy to fix/restore with just $20-50 ( $100-120 if needing a new bulkhead connector ) instead $550-600 for a full harness. But most of them just need mantenience at bulkhead terminals and fuse box clips. Diff story is the engine harness, NSS, which with the oil and heat gets all wire covers cracked.
Oh and about the accesories harness such as AC, heater, 4 speed, A01 light package... they are separated harnesses even being linked to it, so when ordering they are not included. Can't tell on earliers, but on 71 and laters, the underdash harness gets the wire source coming from fuse box free hanging around ( a packard terminal ) to be inserted either into the HEATER plug harness or AC harness plug. I think earliers gets the Heater and AC harness with the wire to be conected straight to the fuse box.
