It's been a long time since I worked on a Fusey wagon, so don't quote me. The motor should be like a regular door one. The last one I did was on a Formal wagon, but they didn't change things much.
You'll need to pull out the regulator to get the motor out. I prefer opening the doorgate as a tail gate to do this...that way you won't be fighting gravity with the glass. To do that you need to first open it as a door, then pull the inner panel off.
Inside, you'll need to find a metal rod, like the ones that actuate the inner door mechanisms. The rod you wand run vertically to the bottom of the door. It's job is to prevent the door from being used as a gate when the glass is up. Once you find it get inside the car with a buddy outside the car to help.
Once inside with the inner door panel still off, push down on the rod to simulate that the glass is down. Then open the door as a tail gate. Use your assistant to help gently lower the tailgate because with the glass up the balance is off and it will want to crash down, likely damaging things. You'll need to crawl out one of the back door to avoid smashing the tailgate glass.
Once you have it open as a tailgate, it will be easy for you to do the replacement.
If you need pics, I have a parts Formal tailgate that I can take pics of- but you'll need to wait until the weekend when I'm off.