This is probably a booster fan to try and get some extra heat through the normal heater ducts.
The normal VW heating system is a once-through system, from the engine heat exchangers to the cabin and then out vehicle.
Some guys have tried re-routing the cabin heaters so they recirculate cabin air using an electric blower (rather than the engine cooling fan which is only once-through heating), or it could simply be an in-line fan to boost the movement of warm air from the engine fan to the cabin - you should be able to tell from the air-plumbing arrangements around the fan.
Rob
Rob and Dave's aircooled VW pages
Repairs and maintenance for the home mechanic
http://www.vw-resource.com