Corrosion - I had the same thing on my 560sel. The rotor was really on tight. Try to get some penetrating oil up under the rotor and carefully try to put back and forth turning pressure on the rotor while pulling up on it. Don't be a bear in a china shop - just take your time and it will eventually come off. When you get it off you will find the distributor shaft under it is rusty and corroded from moisture. Clean the shaft with very fine sandpaper and put some vaseline or other lubricant on the shaft before putting the new rotor on. If you want to speed up the process - or if yours is really on super tight - cut the old rotor in half with a dremel to remove and clean and lube as above. Sounds harder than it really is. Good luck!