resistor looks like this .... ballast resistors can go bad unexpectedly with no warning, and the symptom you cited of the engine starting to run until you let off the starter is THE classic symptom of a bad one - the resistor is bypassed while starting by a wire from the starter [to the coil or the output side of the resistor], but when you let off the starter you no longer have that current to the coil, it must pass thru the resistor, to drop the voltage as Benji says - if the resistor is bad, suddenly no juice to the coil, engine dies .... because the resistor was only on originally 12V trucks, yours could have been put somewhere other than the standard firewall mount location, if it's not immediately evident track wires back from the coil to find it, if you don't find one you need to add it
unless you have a coil with internal resistance
Bill
