You need to remove the rocker with a heavy screw driver. Just pry up on it and carefully move it sideways on the ball stud. I just give it a tap and let it pop off. You have to remove the small coil spring first.
Use a socket and just undo it. If it's really chewed up you might have to go to a 13mm and pound the socket on. It should turn out.
Failing that, I suppose you could remove the whole assembly although they sometimes pull some of the threads in the head out. Not a good feeling when that happens. You can tell there's a problem because it will crack loose but turn really hard. Might as well stop and put it back.
Worse case senario would be to drill it out. Don't go there if you can help it.
Dan Caron's
SL Barn
benzbarn@ebtech.net
slbarn.mbz.org