run-on with MSD 6AL

Sounds like there is a problem with the switched power running the MSD box. The box is wired
Small red wire - on/off for MSD
Large red and black wire for +/- from battery
Gray wire - tach
Violet / green - distributor pick up
Orange / black - coil
There is a few other wires.

I have the on/off wire ran to a switch on the dash for the ignition. Or you could also put a in-line switch (two pole) in under the dash somewhere as a hidden kill / theft deterant switch.

The best would be to chase the wires and find out why the switched power is not turning off the MSD.