This is one of the better articles on heating systems, my bet is either the engine compartment relay is out or the blower itself.
http://www.type-911.co.uk/images/exhausts/
go to this link and scrol down to the below files
heatingtech1.jpg 28-May-2002 14:42 306k
heatingtech2.jpg 28-May-2002 14:42 317k
heatingtech3.jpg 28-May-2002 14:43 288k
heatingtech4.jpg 28-May-2002 14:43 352k
heatingtech5.jpg 28-May-2002 14:43 224k
heatingtech6.jpg 28-May-2002 14:43 132k
Bob