when i was a kid i had a VW bug. i wired a tach into it and strung a wire through the cab and back to the engine. as you know the pick-up wire runs to the negative side of the coil. one day driving along, when i would stab the throttle and the car would start to buck. when i eased into it the car revved smoothly. it took me about a week to find the problem. the pick-up wire had gotten squished between the seat and seat track. it was then that i came across my favorite anti-theft device. an alarm and installation was much more expensive than a toggle switch and a roll of primary wire. so in all my cars i used to interupt the tach pick-up wire to a small toggle under the seat or under the dash and over to a good ground. switch off; engine runs and tach works. switch on; the negative side of the coil is grounded and the engine will never spark with no ill effects to anything. anyone can pop a hood and hot wire a motor from the battery to the coil and push start it and drive off. i had heard of people that would steal VW's by pulling off a tail light lens and running a wire from the tail light bulb to the coil so the could turn the car off by turning off the lights. when they wanted to start it they would turn on the lights and push start the car. but, with the coil grounded they would never suspect a thing, and could try all the hot wiring shenanigans (i would have said tom-foolery but that started a riot last time) they want and never get away with my car.