Instead of using a voltmeter, use a 12 volt test light
. The test light should NOT have an internal battery. It should have a light bulb that lights up when connected to the aircraft battery. The wattage of the test light should be 3 watts or less. Any larger than that risks blowing the 1 amp fuse, if it is not already blown, which is a good possibility.
Does the tester illuminate when connected to the voltage regulator output from the switch and fuse module?