Ok guys , when you are testing voltage , whether with a DVM , or the hated test light , you need to remember that not only does electricity flow out from your battery , but for the component to work the electricity has to flow back to the battery. That is why everything is grounded. The flow through whatever component is what makes it operate, and the AMPS are how much of the available electricity is consumed. So when you test for voltage on a ground wire there is a very good chance that your tool is completeing the ground circuit and that is why you get a reading.
The good side of understanding that is that you can use that same theory to test the ground side of your circuit. Say your headlamp is dim , but when you check you have battery voltage there. Put you test light from a known good ground to the ground post on the headlight. If your test light glows , you know that you have resistance on the ground side and can work from there. Electricity always takes the easy route , and if your light has an easier path to ground that the ground wire it will flow through your test light.