Duster Headlights not working even with power?

If the ground jumper from the battery is properly connected to the core-support, then those other grounds do not affect the headlamp circuit,. But if it's not, THEN the only way back to the battery is thru the firewall to head jumper, thru the engine,to the ground cable, to the Neg. post.
Take a headlight out and jumper the pins to the battery to make sure the dang thing works. If it does then stick it back in and run a ground jumper back to the battery. If it still doesn't work, go back to the bulkhead connector and look for a warm wire; you may have to go all the way back to the dimmer switch light-green supply wire; or the headlamp switch black with tracer supply; or even the #1 splice 12ga black supply from the ammeter guage; or back thru the bulkhead connector still 12ga black,.........
Or just go straight to the bulkhead connector; it's almost always a hi-resistance connection there, or at the headlamp sw itself..
The wire there in question is the big fat 12 ga red from the fusible link,from the battery. On the cabin side this goes to the ammeter, thru it, turns black, and then sneaks off to the #1 splice; then to the headlamp sw then turns green to the dimmer; where it splits into two 16ga wires, a violet and a red , both going out the bulkhead connector again; and then to the headlamps via the sockets. Then thru the filaments to the core support on the headlamp side; thru the core support to the battery via the short usually black jumper sometimes screwed to the engine side of the core support.
If your battery cables are not bonded like the factory cables ,but instead are clamp-on jobber ends, then you may have to disassemble the clamps and clean out the corrosion. The cables are copper and the clamps are supposed to be lead..... which in the presence of moisture, creates a bluish-green compound that can act as an insulator. That is the copper's way of rusting. iron/steel turns red/brown, copper turns greenish-blue.