Dash lights not working on 73 Duster

Fuse box is not chassis grounded. The headlight switch isn't chassis grounded. A new headlight switch will have another male spade on the back of it for a ground wire that is req'd in some models.
The inst' panel does require chassis ground, so if you have removed mounting screws around its perimeter, the dash light would not work.
Voltage at the orange wire will vary with dimmer rheostat but... You should have closer to battery voltage on that tan wire. Check the terminals marked BATT 1 and BATT 2 on the headlight switch connector. If BATT 2 is low, the voltage drop may be in the ignition switch connector. If BATT 1 is low the voltage drop may be at/in amp gauge, or bulkhead terminal connections, or fusible link connections.
Get full 12 volts inside the cabin and everything will work better from wipers to spark ignition to license plate light.