In addition to what the others have said, be careful when you re-insert the new motor shaft through the firewall, The shaft passes through a rubber grommet which is supposed to keep water out. Lube this grommet a little so the shaft goes through it smoothly. I pushed mine through without doing this and started a small leak.

It's much easier to reconnect the crank to the motor shaft if you have a helper turn the wiper arm a little while you match the crank opening to the shaft.

Try that ground strap idea first. I found my ground strap was nearly corroded through and added a new ground wire connected from the motor to the chassis. My wiper motor had a loose magnet. I tore apart the motor, reglued the magnet, greased the gears, put on the new ground, and the wipers are still wiping years later.