Look carefully for a little metal tang under there you may need to pull it out (toward the firewall in this case. Instead of pulling on the arm pry with a small open ended wrench large enough to fit around the motor shaft but small enough to apply pressure evenly to the base of the wiper arm where it cups the shaft. Make sure you don't damage the body work around the area sometime a piece of would or another spanner needs to be used to spread the load. The trick is to apply pressure equally, if it starts to skew on the shaft it will jam (this is why pulling on the arm works so badly, what with the hold down spring and all it is impossible to apply equal pressure. BTW there are more extreme ways as well but this should do it, a little heat from a propane torch may help as well, very little though if you don't want discoloration.