Thanks, Bob.
I learned (via Dave Gallon) that the way to undo the wiper arm linkage (under the dash) is with a battery terminal puller.
You can buy this tool at an auto parts store. Do not get the cheap version for $7-10, get the heavier version for $30.
It clamps onto the wiper arm, and you direct the threaded portion up against the seated ball. Turning the thread pops the ball out, separating the two arms. Take care to line things up correctly, as you cannot replace the plastic grommet if you break or damageg it ... you have to buy a new arm/assembly.
You want to buy the better quality puller as the head that pushes the ball out on the cheap version is quite poorly made and much larger in diameter - and will likely crack or break the plastic grommet.