it will be part of the wiper arm fandangalry and you may have to replace the whole lot.
to do this you take the wiper motor out.
take that black plastic bit off and anything else until its exposed enuff to get your hand in, then undo the knob 3 bolts and the other wiper knob will be held in by 3 bolts as well.
undo them and take it all out and replace the lot.
or have u considered cutting a bolt and then welding it onto the existing stub, then tapping any extra thread over the weld?
might be an easy fix, talk to a local welding/engineering/machinist type of guy.