As you are sure it is a main ground wire, find a suitable position on the frame, clean to shiny metal and bolt it down, coat the repair with a waxy substance.
Something like this CRC 06026 Heavy Duty Corrosion Inhibitor, 10 Wt Oz, Automotive Care - Amazon Canada
Something like this CRC 06026 Heavy Duty Corrosion Inhibitor, 10 Wt Oz, Automotive Care - Amazon Canada
