You could also run something to touch to the assembly to ground it as a test to see if it's the ground. I did that to a few things before installing. I used auto wire with alligator clips on it. I also used that method to find a good ground for electric flashers that needed to be grounded to something. Easier because it gives you a length of wire to work with and you don't need to disassemble/assemble something all the way to get it to work.