I was lucky to have picked them up. The seller bought the wrong ones and thought these would fit his Gen III.
They are held on mainly by some damn strong two-sided tape, and secured in place by a bracket that is screwed underneath the rubber door seals. There is already a screw there holding the trim of the car in place so no extra holes are necessary. There are also five smaller brackets that just clip into place. All the brackets are then covered once the seals are put back in.
It honestly took longer to put the seals back in than installing the side visors themselves.