Mine was seized shut when I got the van (see pictures in the gallery). I got it open by using a piece of 4x2 placed at the very edge of the sunroof panel (on the sunroof itself), then hitting it with a wooden mallet all the way around, with more attention to the back edge and sides nearest the back as the roof needs to drop slightly before it can move back. This took quite some time and a surprising amount of force before any movement was seen from the sunroof panel. As soon as I was satisfied it was all released from the main body, I attempted to open it, very carefully as the handle's can be quite brittle, by unwinding it a little, but don;t force it as the cables can stretch and snap and are really hard to find. As soon as it was turned a little, I went back outside and gave the rear of the sunroof section a few more taps to ensure it began to drop. Then back inside and continue winding etc etc... As soon as I could see the cables I sprayed liberal amounts of WD 40 into every nook and cranny (mainly the sliding channel and the cables).
It took me about half a day to get it fully open, but now works fine, if a little stiff from corrosion on the sliding channels.
Hope this long winded explanation is of help?
Dan