Jet,
If the GMC is like the Chevrolet truck: on your driver's side look under the dash and see where the driver' side mirror attaches through the cowl - two holes in/through a brace.
Look under the dash on the passengers side and you will see a similar brace with holes (I hope). Punch up at the center of each hole, and then drill for the mirror screw holes.
PLEASE READERS: if I am wrong quickly correct me. I'll check this on my GMC COE tomorrow.
