Here is a schematic for your vehicle, it's a bit different from the '72 and later "d" pickalls.
Use this to trace out the wiring, that's the way we do it, there are no magic bullets. Work on one circuit/load at a time, don't shotgun this.
All motor vehicles use only a circuit breaker for the head/tail light system, never a fuse. It also incorporates a rheostat for controlling the intensity of the dash/instrument lamps, it gets hot as it's a variable resistor.
If the dash and dome lamps burn with the ignition switch in the "on" position, then most likely the harness connector is not installed to the right terminal set. The dome lamp is switch controlled from either the door switches or the headlamp switch and will always work with the key switch off if the door switches operate correctly (not all vehicles had door switches) and the headlamp control is operational.
Someone has definitely infected your vehicle with the po virus, that's very common on this stuff, I've been fighting a '73 1110 all day. Your description of the electrical issues tells me someone has really botched it.
You should have left the dash assembly out and connected, that way you can trace all wire runs using the schematic. You can run the rig with the dash pulled out if you simply disconnect the speedo cable.
