I have seen it done two ways:

On one boat the mast wiring exited from the base above the deck through a fitting then under the deck it was spliced at a junction box underneath the headliner and then ran across the cabin top and down the inside.

On another boat the wiring continued down the compression post and exited at the bottom to a junction box at the base of the mast. It essentially was like a keel stepped mast.

In both cases a junction box was used to make the splice.

I have also seen a multi wire connector instead of a junction box. Uses less room and is more compact.

David