I'm not a big Sa mechanic but electrical troubleshot cadillacs for years. I think what Tyler is saying is check your wire from the mag to the switch for a short to ground. Seems if you disconnect it from the mag and the switch then put an ohm meter on the wire itself to check for a short to ground. One lead on the wire in question and the other to the frame(usually battery ground works well) and your meter should read open. If it shows an ohm reading then your wire is shorted to ground somewhere, i.e., pinched by a bolt, rubbed thru shielding and touching ground. If your wire shows no resistance to ground then you know it is ok. Not sure on how the mag works entirely but if the lug on the mag is depending on the switch to activate the ground I'd leave my wire disconnted off the mag, put your meter lead on the mag lug, then other on the engine, frame(ground) and check for a reading. If lug is supposed to be isolated it should read open or infinite. If your reading is open, no resistance to ground then I say not a mag issue. I guess you could also do this to the switch, check and see if it is stuck in the position allowing ground all the time, again, not sure on how this switch works but if it is actuated somehow to ground and is stuck in that position it would be grounding out the lug on your mag all the time. When you toggle your ignition switch does this somehow disconnect the ground to the mag? Now, I'm curious to hear how this thing works myself.