E26 Wiring at mast step

I have dropped the mast and installed a deck light on the mast of my '84 E-26 (#256). There is a 4-pin connector on a wiring harness coming out of the deck, sealed in with sealant, to mate with the connector into the mast harness. Luckily, there is a spare pin on the connectors to accomodate the fourth wire needed for the deck light. Unfortunately, only three wires come out of the deck. How should I get a fourth wire from inside the boat, through the deck inside the deck plate? The deck plate seems to sit right over the mast support column at the corner of the head enclosure. I don't want to blindly drill through the deck for fear of cutting wires, etc.

Any ideas?

Thanks.