The mirrors mount on the cowl, forward of the doors. Look under the dash. There is a reinforcement piece with holes in it. Drill through those holes to the outside.

"It's just a phase. He'll grow out of it." Mama, 1964

1956 Chevy 1/2-ton 3100
1953 Chevy 6100 "The Yard dog"
1954 GMC Suburban Now with a new proud owner.