The issue is feedback. Something feeding voltage to the coil. Your test is valuable. But most folks are following the "instruction" to run the wire to ignition "ON".
So without knowing what actual switch you have, what harness you have and if you wired it per instructions or wired to the ACC fuse yourself, the bottom line is that it's impossible to say if you have a feedback path when switch is off, at this point.
My truck does not have a feedback path when the switch is off because of using the ACC connection for the exciter wire. My ACC terminal is not common with the IGN terminal when the switch is off. I think the reason instructions would say run the wire to ignition "ON" is so that one could use the radio or other low current accessories without the motor running and without draining the battery through the biasing circuit of the alternator. I was not worried about that and used the ACC connection to prevent feedback to the coil. With the side marker bulb in series the drain is very small and has never been a problem.
Mark
