drill a small hole in the mast cap toward the front (far away from the halyard sheave area) would be my first suggestion. If you intend to find your leak and fix it, patch the hole with some fiberglass. If you intend to leave it leaky (or want the possibility to drain it in the future) tap the hole and insert a nylon screw with o-ring with a little sealant.
Actually, you may need two holes - one to vent and one to let the water out. drill a tiny one on the side of the cap about mid way up the mast profile.
