Not if you have a seperate wire (other than the probe) coming off the multimeter to a seperate ground..... Do you recall if that's the case or not? On my tester I have an aligator clip that goes to a ground (in my case I ran a long lead to it so I could reach anywhere in the boat from my neg terminal on my battery,) and then the pointed probe that can be pushed through wires to trace the charge. So if my probe showed a reading at a fixture, for instance, but I couldn't get the light to work even with a new bulb, by process of elimination it must be a bad ground.

I'm not familiar with the Perko fixtures... If you can trace the wires leading out of the fixture, examine the contact point for the black one, and if possible use it for the ground wire on your multimeter.

What a pain, this is something that would take 5 seconds to demonstrate in person, but it reads like stereo instructions on here.