2000 Cadillac Deville Misfire

Is it a dead misfire? In other words, does the cylinder ever work? If it is dead, remove the plug and check to see if it is wet with gas. If it is, check the plug to see if it gets spark. If there is spark, the injector is most likely stuck and dumping fuel. If it is dry, the cylinder has good compression, and there is spark, then suspect a bad injector.

I believe you are referring to the plug cap from the coil. It goes on the plug itself. And yes, they go bad. When they do, rather than the plug getting spark, often times it will jump through the side of the rubber boot and ground to the engine. Most times, if that is happening, the black rubber will have gray spots on it where it happens.