I suspect that you may have a problem with the ammeter.
Connect your voltmeter to one of the ammeter terminals and note the voltage reading. With the voltmeter still connected to the ammeter, turn on the headlights. What is the voltage reading? Move the ammeter probe to the other ammeter terminal and repeat the test.
If only one test reveals an extremely low (or zero) voltage reading, then the ammeter has a problem. Remove the ammeter and connect the two large wires together, and see if you now have power to the rest of the car's wiring.
If both tests reveal an extremely low (or zero) voltage reading, then the problem lies between the ammeter and the battery.