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Can you pull the coil off and get ahold of the broken bolt with a small vise grips or pliers to turn it out? If the bolt is broken off flush you will have to use an easy out to remove it.
Soak the bolt with penetrating oil (Leave the plug in or stuff the plug well with paper towels so you don't wash junk into the combustion chamber) Center punch the little fricken bolt so your pilot drill has a good start. You drill a very specific diameter pilot hole (pay attention to the pilot hole diameter and USE THAT BIT). Put some tape on your bit so you know how deep to drill. Carefully insert your reverse thread easy-out. Slowly increase torque until it backs out.
Worse case scenerio: drill out the entie bolt, tap it, and use a heli-coil (same watch the pilot drill size rules, along with recommended tap rules apply).
Hope it isn't plug 4 or 8
Let us know how you made out so other members can learn from it.