1975 Duster Potential Wiring Harness Issue

Good suggestions above. Get the fuse panel out and stretched down where you can access it. Measure voltage, under load across the two fuse clips. You should measure almost nothing, zero volts. If that seems OK, Check both clips to ground under load. Simply, determine where / when you are losing voltage

Examine the fuse clips for corrosion, heat damage, rust, etc, and look at the back, wiggle wires check connector tightness, and inspect for discoloration showing heat damage, etc.