It takes amperage to light the bulb, small parasitic draws .050 mA will not light the bulb but can drain the batt. over time,there is a large amp.draw somewhere,use a DVOM to ck.amp.draw so you know just how much actually is being pulled as long it's not greater then the DVOMs fuse,then you will need a shunt,but the good old test light between the cable & batt.works just fine, me personally I would be taking a close look at the no longer utilized lean burn wiring & the elect. ign. conversion.