I think you've posted in the wrong forum. Maybe admins will move it for you.
What you've described is classic
voltage drop. First you need to make sure which side of the circuit you're losing. Because you're at the fixture, odds are you're using the wires there for both the power and ground test. A loss of voltage there could indicate a voltage drop on the power side or ground side.
You'll need to get a ground source back to the battery
. This will mean a long wire from your meter ground lead to the battery
. Once you have a ground, check voltage at the fixture on both the power wire and the ground wire.
If you have little or no voltage at the power wire, start chasing that side of the circuit. Test the voltage at each switch and each connection. When you find the one with good voltage in, but very little out, you've found the culprit.
If you have good voltage on the positive wire, but also have voltage on your ground wire, then the ground side is the culprit. Simply trace the ground circuit in the same way, looking for where the voltage drops from whatever you were reading at the fixture to less than half a volt.