It should just be a remove and replace affair if you have dual mirrors from the factory. If not, see if the bracket is located in the door underneath where it should be (GM double layered the door mirror area to give it strength). Then you could take a small drill bit (or a punch)and drill a pilot hole from underneath and finish it from the outside with a larger bit. If you go a little oversize, the gasket and mirror should cover the hole and make it invisible from the outside. Be sure to paint the holes to inhibit rust.