Locate the build month and year on the black VIN plate on the driver's door jamb. In my car's case it is 11/85.
Locate the PKN code on your car's build sheet sticker in the trunk:

My car's code is 45-4-8063.
^That code yields:
45th week, 4th day of the week in 1985. That translates to: Thursday, November 7, 1985; my car was the 63rd car to roll off the production line at the Karmann plant on that day. That date, believe it or not, corresponds to the date printed on my car's birth certificate.

Complete instructions here: http://www.cabby-info.com/Files/Misc/BuildDate.pdf .
Credit to Brickwerks over on TheSamba for the PKN info.

Edit: If you do not have the build sheet sticker in the trunk, pop your hood and locate the PKN plate riveted to the slam panel (aka radiator support):
It provides the same code.
