amp guage issues need help

i need some help i have a 73 duster and the black wire under the dash is partly burnt.
i was driving the car with my headlights on and smelled something burning.the dash lites dimmed and the amp guage went to neg.i noticed a low amp reading ......how can i repair this or upgrade to a better,and stronger elect amp guage wiring NEED HELP ANY IDEAS ?
i was advised to remove the wires from the amp guage and install a volt guage...need help please :???::???: