Most likely the door latching mechanism, see info in this thread:
http://986forum.com/forums/general-discussions/41706-getting-2002-boxster-advice-needed-2.html
I had the same issue a few months ago and replaced the door latch to fix the problem. There is a micro switch in the latch unit that fails.
There are a couple of things to be aware of when installing the unit; 1) there is a small paddle that comes off the door lock barrel (key lock barrel) that inserts into the door latch mechanism as you install it (easy to miss... don't ask me how I know), 2) the door latch mechanism attaches to the door handle lever mechanism with a clip that snaps over a small threaded rod and then is locked in-place with a sliding clip. It's hard to see these inside the door, a small mirror on an extension will help.