You have to remove the seat cushion assembly from the seat. Probably 2 nuts somewhere under the front edge, several electrical connections and it should come out. The leather has hooks sewn into the edge that attach it to the seat cushion frame. A little tugging and pulling will remove it. Chances are the broken element is in the seat cushion, not the back, and if you find and repair the break you should be good. The elements in the seat cushion are subject to a lot more stress and strain than those in the back cushion, and are generally the ones to fail.
Use masking tape and a pen to mark all the electical connections under and in the seat so you make sure everything is connected back the way it was before you started pulling things apart.
2001 Bonneville SSEi (retired at 365,000 Kms.)
2014 Jeep Grand Cherokee Overland (yes, I know its not a GM)