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Something interesting I found online:

Plain to Carbon Filter

Function:

The cabin air filter's function...well, take a wild guess. :)

The main gripe I had with the filter is the expense of the activated-carbon treated filter (around double the OEM cost of the plain filter), considering that this "special treatment" is a small amount of activated carbon sprinkled in the pulp mix. Fortunately, the space given to the filter is enough to allow some design improvement.

Part Information & Sources:

Note - always confirm part numbers by looking at the actual part mounted in the car! Mistakes will happen sometime, somewhere.

Part numbers:

Plain filter
'98-01 8A0 819 439 A
'01.5-05 1H0 819 644 A

Carbon filter
'98-01 3B0 091 800

OEM Pricing: ~$35-$40 ($65-$75 HEPA/carbon)
Impex: $19 ($30 HEPA/carbon)
Adirondack: $19
A Volkswagen Part: $15 ($27 HEPA/carbon)

Filter removal:

The cabin air filter is located on the rear passenger side of the engine bay, behind the firewall. (illustration)

1. Start by unscrewing the single philips screw visible.
2. Pull out the filter lid - this piece prevents water from flowing onto the filter, as well as directing airflow. (illustration)
3. To remove the filter, lift the exposed portion up and pull out at an angle. (illustration)
4. Take a flashlight and look into the filter housing - some leaves may have fallen into the blower duct during removal. Pull out anything you see that isn't plastic, unless you enjoy the whirring sound of leaves being blown around (I have a friend who does, actually).

After two years and 30,000 miles, this filter had served its useful life. (illustration)

At this point, you can either stop here and just pop in the new filter, or if you like to live dangerously, keep reading...

Activated carbonization:

5. Swing by your friendly neighborhood hardware/general store and pick up a carbon prefilter designed for use with home HEPA filters. Home Depot carried a generic cut-to-fit Honeywell sheet for around $10. Fairly thin stuff (1/8"), but far more useful than the combo cabin filter/carbon mix.
6. Cut one or two pieces to fit. Using two pieces per filter, you should have enough carbon to make three more carbon changes, or seven with a single piece.
7. Attach the pieces to the filter. If using one piece, attach to the top of the filter. Note inelegant use of duct tape...ah, well. (illustration)

Note:
Using the carbon as a pre-filter is preferable because a) carbon particles that shake loose will fall into the main filter instead of being blown into the cabin, and b) carbon sheets are far less expensive than the paper filter. The carbon sheet will catch quite a bit of material itself and should extend the useful life of the paper filter.
8. Reinstall the new improved superfilter you've just created. Be careful when reinserting the filter - it's a pain enough with the paper filter alone, the carbon sheeting just adds to the joy. Have a helper pull on the plastic gutter above the filter housing, this should give you enough room to slide the filter into place without scraping too much of the carbon sheeting. Properly positioning the filter requires some filter acrobatics, don't be afraid to bend the filter slightly to get it into place and completely seated down.
9. Take additional care when reinstalling the plastic filter lid. Push the lid upward (using a hand underneath) as much as possible when sliding it over the carbon sheeting to keep from catching the sheeting and moving it aside. The lid fit fine even with a layer of carbon above and below the paper filter.

All done! Take your baby out for a drive and smell that carbon freshness. The blower motor is more than powerful enough to deal with the added load of the carbon filters - there was no notable change in airflow with two pieces in place. The replacement interval on the carbon for home use is around three months, though the useful life in this application will depend on your area. The OEM carbon filters are spec'd for yearly replacement.
 
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