Nice ride! And I am not sure about Air Cooled, as I am a Water Cooler, but this is what I would do in my car. Go behind your fuse panel and there should be a mess of wires find the main trunk comming in from your battery and put a volt meter on it if it spikes you have power comming thru.
If it is different in your beetle than it is in my car you can just start tracking wire groups and see what they go to then use your volt meter to see if the end station is hot. I had a similar problem turned out to be a light that was on with no bulb ergo you could not see a light source so it would stay on and drain the battery, missing any bulbs in lets say a lower door lamp, or the trunk space?
One of the Air coolers may have a better or less time consuming way of tracking it, and if so will spit it out I'm sure.
Nice to meet you.
Just curious real quick: How did you find us?