I bought a few "alternative" switches in electronics surplus stores many years ago, knowing the high current on top speed destroys the switch contact eventually.
If you open the switch by spreading the rectangular case and fingers, you could probably rebuild the worn contact face with a solder-blob, and then have THAT contact pull in a relay to feed the fan directly on high speed. I noted that GM had done exactly that in a 78 Gran Prix I owned briefly 30 yrs ago. They made a heavy gauge link from the battery to the fan with only the relay in the way for "full blast" operation.
Other speeds went through the bulkhead and variable resistances.
Check your fusebox terminal clips for the fan, too, if you have a "toasted" fan switch.
Probably heat oxidized, weakend clips.