i think you can get just the micro switch insted of the whole assembly.
Pelican Parts - Automotive Parts and Accessories - Porsche & BMW
there is another one located on the external door lever mechanism.
you will be able to diagnose it better with the door cover removed.
you might be able to solve the issue by simply cleaning it with electronic cleaner spry.
edit: links for parts.
Micro switch on door lock mechanism (part 20):
http://www.********************************************.com/porsche-parts/986-97-04/807-10.php
Micro switch on door external lever (part 14):
http://www.********************************************.com/porsche-parts/986-97-04/804-20.php