Use your tester, and get the voltage from both sides of the ballast resistor (72-up have the 4 prong deals...) You will have two inputs, and two outputs on it., The two inputs should have bettery voltage at them with the key in the run position and start position. One leg of the ballast resistor has a higher resistance than the other. This is the "run" leg of it. The "start" side has less resistance in it. When you tunr the key, the "start" leg should have close to battery voltage present (disconnect the fender relay for these tests so the engine can't turn over). WHne you release the kay and it springs back to the "run" position, the other leg of the resistor should show about 9 volts on the output side. If it shows less, and the battery voltage is right on the input side, it's the resistor that may be incorrect, and you may find it hard to get the right one. If the batery voltage is low on the input side, there is a problem between the battery and the ignition switch, or between the ignition switch and the ballast. It's fairly coommon to see the brown wire going to the ign switch to be melted a bit, and the connectors in the plugs to be melted to, because a lot of guy use the key to turn the engine over for far too long.