Engine light on. went to auto parts store, they diagnosed number 2 or 3 cylinder not firing. prior to this i changed plugs and wires. then i changed the coil. the engine light went off and the gas economy increased. then the light came back on. talked to someone at pepboys and he said there is a wire that comes with the spark plug wire kit, like a ground wire? i kinda sorta remember seeing one in the box, but ignored it.. is there a wire that comes in the plug wire kit? i'm ready to buy new plugs, plug wires, and a coil. am i on the right track?

Hello mikew564.The first diagnosis was number 2 or 3 cylinder not firing. Once you changed the coil, the check engine light went off. When the light came back on again, did you get it diagnosed again to find out if the problem was the same?... 2 or 3 cylinder not firing????

First of all, there is NO extra wire/ground on the coil/ignition wiring (see images below). The spark plugs and ignition plug wiring can be checked for arcing, corrosion, resistance and can be checked for cracks in the insulators or worn electrodes (and spark gap) by a competent mechanic.
If you are having the same issue (2 - 3 misfire), there are several techniques for eliminating the plug wires or spark plugs. Spray water on the plug wires to see if they are arcing. Swap plugs from cylinders 2 & 3 with cylinders 1 & 4. If you still have a 2-3 misfire, check/inspect/examine the coil connector pins & coil harness to the coil(s) themselves.