Every good rain I get some water coming into the rear light assembly on my 2001 OB. The bulbs short out, and water comes dripping down when the hatch is propped open. Is this a common problem? And what are some likely paths that the water would be getting in?

I've had a leak (in my 2002 Forester) with water getting in through one of the mounting bolts for the overhead longitudinal roof-rail.

And this leak was hard to find, because the dripping water was hidden behind the interior wall-panels of the car.  The leaking water was evident on the trunk-floor, and in the well for the spare-tire.

However the water never entered the rear light-assembly. That unit is virtually sealed all around, except where the bulb is fitted.

This is a common problem on 2000-2004 Legacy and Outback wagons. The center garnish ends up filling with water running down the hatch. You can either put a bead of silicone between the rubber gasket of the window and the top of the garnish. YOU can also pull out the garnish and lay silicone all the way around where the front and back plastic sections of the garnish meet. Check the sockets for melting or corrosion and replace if nessessary. If they are badly melted inside, it can cause some decent electrical gremlins.

What is the "garnish?"

The garnish is the trim piece on the hatch that says "Subaru", which also holds the inner brake lights and reverse lights.

The garnish is the trim piece on the hatch that says "Subaru", which also holds the inner brake lights and reverse lights.