It would help to use the tool made for the job. I believe it's called hose pliers or spark plug boot pullers. It looks like pliers with the tips curved into a circle that helps you grab the spark plug boot, give it a twist to break the bond with the ceramic. This is your best chance of taking it apart. Of course the boot may be hopelessly baked onto the ceramic and you'll have to buy a new boot. The technique I use is to first break the bond then move up and grab the boot where the end of the clip is inside the boot for removal. This prevents ripping the clip off the ignition wire end.

The best way to deal with the problem is prevention. Buy yourself some silicone grease (dielectric grease or brake caliper pin lube). Every auto supply house should have some. Butter the inside of the boot where it contacts the spark plug ceramic with silicone grease to prevent this problem in the future. As long as you're at the store, buy some anitsieze compound for the spark plug threads.

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95 E320 Cabriolet, 159K