You have to put a diode in one of the wires for the voltage regulator. It is shown in the MSD manual.
ENGINE RUN-ON
If your engine continues to run even when the ignition is turned Off you are experiencing engine Run-On. This usually only occurs on older vehicles with an external voltage regulator. Because the MSD receives power directly from the battery, it does not require much current to keep the unit energized. If you are experiencing run-on, it is due to a small amount of voltage going through the charging lamp indicator and feeding the small Red wire even if
the key is turned off. Early Ford and GM: To solve the Run-On problem, a Diode is supplied with the MSD in the parts bag. By installing this Diode in-line of the wire that goes to the Charging indicator, the voltage is kept from entering the MSD. Figure 6 shows the proper installation for early Ford and GM vehicles.
Note
iodes are used to allow voltage to flow only one way. Make sure the Diode is installed facing the proper direction (as shown in Figure 6).
Ford: Install the Diode inline to the wire going to the #1 terminal.
GM: Install the Diode in-line to the wire going to terminal #4.
Figure 5 Wiring the Dual Ballast Resistor
Hope this helps