GREY NOT TO TACH,tach plugs into the slotted wire plug on the msd box

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Don't know about a "slotted wire plug". The box has only one plug-in for the wiring. The instructions say the grey wire to tach. I just hooked it to the factory wire at the tach. Tach works normal hooked up this way. I don't know any other way to hook it up. Jeff