I have no idea if it would have enough amperage for the sunroof motor, but I've used a regular 9v battery to test auto door lock actuators and window motors, granted the 3v difference causes them to move really slowly, but it might let you know if it's the motor.
Otherwise I guess the option would be to either pull the battery so you can get it closer to the sunroof or pull the motor to get it closer to the battery. Or jump from somewhere in the cab, like the cigarette lighter.
Or try bypassing/jumping the sunroof switch. Have you got a multimeter with which you can test for power at the switch?
Also did you check for the bad solder point they mentioned further down? I've had to do similar things to both my headlight switch and GDM, with great success.
Sorry if you've already tried all this.