I have taken apart a few GM rear wiper motors and they are pretty robust, but that wiper arm has a splined shaft and usually the wiper arm gets smoothed out. I would look at an auto wreckers and buy the arm, it is held on by a 14mm nut. You will need a piece of wood and some kind of prying device ( the wood goes on the tailgate so you don't scratch it) then wiggle and pry off the arm and replace it. You should be able to hear the turning, before you go for the new arm. If the motor is not turning it usually is that the shaft is seized, it is an easy fix. Take out the torx screws and remove the plastic cover. Now you can turn it over and mount it on the aluminium case and drive the shaft out, yes it is stuck but it will come out. once out emerycloth the shaft and with a rat tail file clean up the bushing area.then grease it up and put it back together, that big plastic gear only goes one way and the shft has the center hole and the other one to rotate the shaft. Remember not to go crazy with the 7/8" nut that holds the unit from the outside, and again there is a slot so it only goes one way. this whole thing should take less than an hour
