On the lower side of the cap, there are two holes right behind the mounting screws..
In fact that was my original issue... some genius had filled them with caulk!
I got the dremmel on them, and opened them up on the inside to allow water to flow out right from the base. Kinda like a port and polish job!
You can just about make out the drain holes in this pic... in the screw recess, behind where the head of the screw will be eventually.
More visible on the unpolished vent..