It sounds like something is keeping it active or it needs a change in voltage to deactivate. I'd suggest finding some schematics and see if you can learn more.
It's very easy on an mt82. The factory wiring is already in place going to the trans. Find the pin out, grab the two wires and connect them to a switch on your shifter.
It's been a year living in Alaska now. We love it, but I do miss the mustang. We live right near the only nhra 1/4 mile drag strip in the state.
We've been building up a jeep to go adventuring when we're not on the sxs.