Further to my earlier comment, some years ago I had exactly the same problem that CatGra has, and in that instance I had twelve volts as measured with an AVO Meter at each of the lamp holders in the the deck flood/Steaming light fitting, and two working light bulbs, but no light coming from either of them when switched on. The problem was down to corrosion on the end of the negative copper wire at the under deck coupling just below the foot of the mast. The corrosion was not sufficient to stop the volts from going through to the light fitting, but was sufficient to stop the amps from getting through such that the current measured and found to be present at the lamp holder, was insufficient to light the bulbs. The fix was to clean the ends of both the negative and positive wires at the under deck coupling and tin them with a soldering iron, or replace them with tinned copper wire.