It wouldn't make a difference if it was 6V or 12V for testing. It just momentarily hooks one wire to another, or a battery to the part you want to test. I have a couple of them. It's just a momentary switch, with two wires, one to connect to power and the other to connect to whatever you're testing. like the one Paul showed us a photo of. 6V does require twice the amperage as 12V which would require larger wires but if you're just momentarily testing something a 12V test or should work fine. Except, it wouldn't work for the main power to a starter, it would go up in smoke along with your hand holding the tool, but for everything else it should be okay.
If you think you have a wiring or connection problem the best thing is to disconnect the battery and do a voltage drop test. This will show you where you have excessive resistance in a circuit. YouTube has several examples of how to perform the test. It's what I used as a mechanic for 30-years to find resistance issues in circuits. You just need a voltmeter. It will pinpoint where the resistance is in the circuit. It's a really neat test and will save you from replacing good parts.