I went thru this...ended up using a Vise Grip to get that puppy out. I installed a new one with a thin layer of anti-seize, so it shouldn't ever be a problem again.
Plus now I use a topsider!
"If everything seems under control, you're simply not going fast enough" --Mario Andretti
1995 E500 street car
1986 Porsche 944 Turbo S track car