My suggestion, twist the "boot" at the plug to loosen it on the plug then grab hold of the metal shield and pull on it. I effed up once by not doing it like that when I was just going to change the plugs and I grabbed the boot, my hand slipped and ended up pulling the wire and "wasted" it. Ended up with a new set of wires that day
Here are some pics of how they all look like after being pulled out. (Just went to the garage to take one out so I could post the pics) I hope it helps explain how the set up works. The metal shield will pull the boot from the end.
Good luck, Ed
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