The part of the base gasket facing the rear of the car has a small tip up point in the middle. Pry out this "tip" and you will see the metric allen set screw. It is about 1/8 in size, but is metric, NOT US standard. You want to locate a proper metric allen to use. If you strip out the head, it will be a B**** to try to get it loose or tighten it. Mine were sripped out when I got the car, and I had to eventually buy a used set to get undamaged set screws to tighten. As Eric said, the nuts on the inside of the door are welded on - stay away from them. Not only will they not solve your problem, but if you could somehow break the welds trying to remove them, you would not be able to attach the mirror to the door at all. They would have to be re-welded, with accompaning repaint of the door to repair the weld damage done to the paint.
Best Wishes,
Robert
1988 Black Spider Veloce