Use a volt/ohm meter and with the car running measure from the back terminals of the alternator if you can get to them in your car. If you have voltage coming out, go to the input to the converter and see if it is there. If you have voltage going into the converter and none coming out, then you have most likely found your problem. Normally the battery should be reading about 14/15 volts when the car is running so that is what should be coming from your converter.
Ron
