I would again treat with rust buster let it soak overnight then try it it a hand or air impact wrench and allen wrench bit.
If that does not work I would remove the AFC and put it on its side and drill it out or try loosening it with a punch But would not drill it out on the truck YOU KNOW WHATS UNDER THAT PLUG good luck also I might try applying heat from a torch to it. The housing should expand differently from the plug.
Good luck
If that does not work I would remove the AFC and put it on its side and drill it out or try loosening it with a punch But would not drill it out on the truck YOU KNOW WHATS UNDER THAT PLUG good luck also I might try applying heat from a torch to it. The housing should expand differently from the plug.
Good luck