That depends on where you have the ground wire of the test light connected. If it is on the ground terminal and there is not a good ground connection the problem is ground - not missing 12 volts. Current can't flow to light the light if there is not a complete path to ground. So the long and short of this is that the problem can be 1) missing 12 volts, 2) missiing ground, or 3) missing tach signal from the engine (S-termiinal on the gauge).
