89 Cobra LX, I'm not sure how you wired your ignition, but from what I know, you usually just put some wires between the coil and the stock coil plug. The stock coil plug signals the MSD box and then the MSD box fires the coil. You need to probe RPMs from the back of the stock ignition wire. It's the dark green and yellow wire. Or you can just hook up to whatever the white wire out of your ignition is plugged into.
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351W 89 Mustang GT Convertible