I believe the motor and gearbox are a unit (sealed of course). That's why the $$$$.
Try this before committing to the $: pop the center cover off and move it out of the way. A small mirror will help you locate the 2 slots on the underside edge. The other cover near the bottom of the arm releases easily too. Sorry, before doing this, move the wiper through a half stroke manually, so it is completely extended. Now that you have the 2 plastic covers out of the way, you should see a piston of sorts in the middle and 2 guide rods. Lube them with a good synthetic grease, perhaps one intended for snowmobiles, as it will have the winter weight you need without getting thick and binding up. Don't overdo it. Move the wiper back to the park position and back up a couple of times. Put the covers back on and park the wiper manually. Try it with the power now. Hopefully, that was all that was wrong. Do this in the summer too to make sure that any dirt is kept out and to make sure your grease hasn't liquified in the heat and run off. Do it again in the fall. Best of luck to you. I got lucky with mine last winter. It's still OK (knock wood).