Well here I go. Get a decent thick walled socket, go find a friendly gas station and ask them if you can either use or let them take a shot at it with an impact air wrench. If this doesn't break it free, then it probably will never come out and you'll need to see a dealer. If it remains stuck, you're probably looking at some sort of destructive drilling and the like. The air wrench has always helped with those difficult cap screws... Good Luck..
1987 BMW 325