Fitting a Blower to air-cooled VWs
I got a blower from a local boaty shop - the blowers are used to ventilate boat engine bays and bilges to get rid of any fumes before starting. One website I found with the same kind of thing is:
http://www.jabscoshop.co.uk/item.asp?sid=B8610F3D922743BFBF20F48067AB0D12&ls=Rule+Marine+Pro&id=1921
or:
http://marinestore.co.uk/Merchant2/merchant.mvc?store_code=mrst&screen=PROD&product_code=pl26949
The blower is fairly noisy on full power, but at least you get a clearer screen more quickly, especially in the winter - I fitted a resistor and a 2 way switch so I can have half power (and much less noise!) as well. I mounted it behind the front grille on the bulkhead - just cut into the air duct and use jubilee clips. The resistor I used was from RS Components - part number 252-2726 and the switch was 440-6991 (I used such a mini switch to make it easier to cut a hole in the steel dash for it - and it's less obtrusive. But you may prefer a physically bigger one). The blower and switch were wired to supply direct for full power and thro the resistor for half power. Take a feed from the fuse box that is only live with the ignition on. I self-tapped the resistor to the bulkhead as a heat sink to help dissipate the heat. The blower draws over 5 amps so make sure you use appropriate cable.
Bilge blower fitted to bulkhead behind
grille in an aircooled VW T3 (aka T25)
Another example of the same kind
of thing (photo courtesy John Windwood)
Switch fitted in dash
near driver's left knee and resistor for half power screwed to bulkhead as a
heatsink. 
Close-up of resistor
Another way to do it is to fit the
switch into heater control panel (idea and photo courtesy John Windwood)