I had this problem last year. As others have said, you definitely need to replace the door lock assembly. The problem is a microswitch inside the latch assembly, but you can't get the switch by itself. I tried lubricating it with no luck and then replaced the door lock assembly. It was a DIY for me.
The part is called the 'Door Lock Mechanism', and is available at:
http://www.pelicanparts.com/cgi-bin/ksearch/PEL_search.cgi?command=show_part_page&please_wait=N&make=POR&model=986M§ion=INTpnl&page=1&bookmark=4&part_number=8N1-837-015-C-M100
I got mine at the local dealer for a few $ more, but it was worth it to get it done the same day.
You will need a special type of wrench called '12 point triple square' to remove the part from your door, but I found a set at the local O'Reilly Auto for not much $. It is used to remove the 2 bolts visible on the door jamb part of the door.
Good luck with the repair!