I don't remember the wiring color code for a merc control but you have to assume the current that is shocking you if it is constant is coming from the hot or positive feed to the trim switch on the control.If it shocks only when you actuate the switch then it is one of the other two wires going out of the switch.Check to find an abraded spot in the wires making contact with the metal on the control. Possibly in the wires wound around the handle? You will have to remove the handle to check.
