Maybe this idea will save you a lot of work.

My masthead or steerage light stopped working and with some testing it was clear that I had an open in the circuit. When the yard inspected the light the only the black wire - (common) was still attached to the fixture. The power wire as unable to be located by any means inside the hole through the mast and inside the PVC conduit running up the mast.

What I didn't do when I installed the steerage light in 2005 is to coat the bulb terminals with dielectric grease and secure the wires so they don't fall down the tube/mast.

I was able to resolve the issue by first securing the black wire at the top of the mast and removing the "lost" power wire, then used remaining black wire to pull down 2 slightly smaller power and common wires along with a pull string.  If you use electricians lube and assuming the PVC tube isn't already full - the pull should go well. 

Thanks to Randy Leasure for being the safety man on my first mast climbing excursion.

Jay