Get a light line, longer than the mast. Unwire the electrical fitting in question. Strip back the cable outer to leave about 3 inches of wires exposed. Twist these tightly together at the bottom, then untwist the 'lowest' twist just enough to get your light line through between wires. Tie the line back to itself with something suitable (a couple of half hitches). Twist the rest of the wires in the cable together, twisting in the light line as you go. Wrap the whole join in tape and roll it between your palms to make it as smooth and tight as possible. Then start pulling the cable through the mast, pulling the light line in to replace it. If it gets stuck, don't just heave on regardless, but pull a few inches of cable back into the mast and try again. Eventually it will clear the obstruction, and you'll have the old cable out, and a piece of light line running through the mast. Then reverse the process to pull the new cable back through. I don't think it matters whether you pull the old cable out from the foot of the mast, or the upper exit, although I've always done it from the bottom. Only change one cable at a time, or there's a slight risk you might get twists between the cables, given the propensity for light line to invent knots at the slightest opportunity.