So if the power wire is "downstream" of the ignition switch, meaning it is turned off when the key is off, try turning on the key to supply power and then reading the voltage between the ground and power wires. Should read approx 12V. Set meter to measure volts DC. (not AC) If it needs a range, try 0 to 50 volts, I don't think the lighting wire would have any effect here, it merely powers the bulb. If the voltage measurement checks out, the signal wire has to be the source of the problem; maybe for some reason your ignition setup is putting out too much voltage for the tach and burning it up? You could try measuring voltage between the signal wire and ground as well. Again volts DC. I think Gary's idea of checking with Pertronix is a sound one as well. Maybe you need resistor of some kind?