As ususal, MoGolf (Glen) provides excellent maintenance advice. Clean your door and sunroof drains periodically (from CinciTDI list):

The drip from the door after a rain is normal. Water will get inside the door past the window seal. There are drain holes at the bottom. Make sure they stay open and don't get clogged with dirt or anti-corrosion sealant that oozes down to the bottom. Use a toothpick or other suitable non-scratching tool to poke into those holes and clear them out.

Due to the angle that the car may be sitting at during the rain, not all of the water is going to drain out. The movement of the door when you open it causes more of the water to find a drain hole and out it comes.

Everyone with a sunroof equipped vehicle should keep the drain channels cleared out periodically. There are 4 of them. The two front ones go to the front door hinge area. The back ones go to different locations depending on model. VW suggests using the cable from an old speedometer cable (removed from the sheath). What you need is something long and stiff enough to push through the channels but not too thick so you can get it through the holes. Some suggest fishing line or weedwacker line of sufficient thickness if you don't have an old speedometer cable.

If you attempt to clear the drain lines with compressed air, you might blow the drain tubes off of the sunroof frame nipples. That will lead to the water draining directly onto the headliner. You won't like that.

---

Regarding the location of the drains for the 04-05 TDI Passats...

 

The front drains are located in the front door hinge area. Just above the wiring bellows between door and body. It looks like a rubber plug with a stub sticking out. The stub actually has a slot hole in the end of it.

 

The Passat sedan rear drain snakes back through the C pillar (sides of the rear window) and down behind the trunk trim to exit behind the rear bumper. Bentley says rear bumper has to be removed to access.

 

The Passat wagon rear drain snakes down through the C pillar too (between rear side doors and windows). Its nipple is behind the rear wheel well liner. The liner must be removed to access.

 

The drain of all vehicles can be tested using a cup of water. Open the sunroof into the fully retracted position. Pour a little water into the outer channel area (righ along the roof panel opening) and watch it drain down the openings at the corners. The water should drain out at the drain exit locations. No water coming out? Then it is past time to clean them out.