Hi,
I had mine apart during a warranty repair that made the problem worse, before finally fixing it (long story). From what I recall the cover fits over the frame like a sock. The bottom part pushes into a slot and the sides clip into the plastic cover on the rear of the frame (hopefully there's a picture attached). Once un-clipped if you place your hand/arm up inside the seat back (between the frame and foam, not foam and cover) you should feel that you can pull it forward an inch or two and find the connector for the heating element. Disconnect that and the foam and cover can then be pulled up and off together over the top of the seat.
The foam and cover are joined together by a dozen or more small plastic clips.
One end of each clip passes through a loop of stitching on the cover and the other end onto a metal rod molded into the foam. Be extra careful trying to separate the clips as there is very little room to get your fingers in - maybe theres a tool that does this? I only mentioned caution as the dealer had ripped part of this rod out of the foam during the repair. The cover would not sit properly after that and the sides of the cover wouldn't stay because the tension from the rod in the foam holds it in.

Hope that helps