Had this problem but on a different car and found out that the fuel tank neck leaked when fueling up. For a day or two after going to the gas station the cabin would smell like gasoline. The car was a 1997 Firebird.
I ran my hand under the car and around the inside area of where the fuel door was (took out the rear wheel well cover) and felt gasoline in there. Slower fueling seemed to fix the problem most of the time. It was cheaper then replacing the fuel tank.
I dont now how you call it, but i searched a translate program, and this came out..
structure attached to the carter which picks up and accelerates the air flow
Check the hose's from that..
If a hose is leaking, then the carter air flow wil come in the interior trough your interior filter..
*** Jeff Edison -- Golf IV 1.6 16V Sportline -- Golf III GTI 2.0 16V -- The Netherlands -- www.VW-CLUB.nl ***