There are two ground straps that are supposed to connect the lower cowl to the engine block. If those are not there and you have stray spark plug voltage leaking, that’s one way to get shocked. Another is if you have an engine with ADI (Alternator-Driven Ignition) or a CDI distributor system, you have a couple of hundred volts in the key switch, waiting to be grounded out when you turn off the key. If you have a bad key switch, a lot of corrosion in the box, or wires with bad insulation (very common in that era of Mercury) you will get zapped from the box. Both of these scenarios can explain the engine misfire, as well.
Long live American manufacturing!
