I would assume that this is a 12V
fixture, so if you are only getting 6V, then you must have a very resistive connection. So step 1, I would disconnect the wires from the light fixture and measure the voltage on the bare wires. If you get 12V
, then the fixture itself is causing the resistance. If still only 6V, I would pull the switch and check the voltage coming into it, and leaving it. You may determine that the switch itself is causing the voltage drop, or maybe you only have 6V coming into the switch, which means working your way back to the next junction. A wiring diagram would be useful at this point.