If you are drilling today, be careful! Use a lot of penetrating oil as prep. And always use a small drill bit to create a pilot hole. You might want to use a drift punch near the edge of the bolt and tap it with a hammer to get it to start turning, just drill a small divot near the edge so the punch has a little grip.

Don't forget, the plastic tree has an O ring and it gets brittle with age.

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My name is Klaus and I am a V♂lv♂holic

Had several bolts with this issue on my 97 all related to the vacuum tree or the bracket for the throttle. Carefully drill them out and repair with nut serts or helicoil and put in new bolts. These are not critical and don't have to be perfect, but certainly use patience.

For sure get it soaked with Mystery oil, and give it time.
Is there anything left that you can grab with vise grips?
Getting it hot helps loosen, but you need to be careful with heat.

Shock helps too, a sharp smack straight on with the appropriate punch might help.

If you have to drill it out, take care to get a small pilot hole dead center.
Don't start with the tap drill size! It will wander.
Clean the top of what's left of the busted bolt and use a center punch.
You could have success with an "EZ Out",
(Please note that they never called it an "Easy Out")

I drilled mine out very carefully with a left handed bit and was able to save the threads so all that was needed was a new bolt.
Soak it with penetrating oil beforehand.
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"Differences of opinions should be tolerated, but not if they're too different' - Sharon Craig

How about using one of those speed out tools that they show in TV!!!