The issue can also be grounding and there is a TSB that was posted long ago that covers the repair. Porsche even issued a kit to do the fix.
My '01S had 4 trips to the dealer to get to where the lite was finally off. First two were seat belt, third was grounding, forth was grounding again with me being told that the zone guy had come up with a new procedure beyond the original TSB.
Fault codes for the airbag system can be read by those with the right equipment.
A writeup by Maurice and the original TSB are
here.
Make sure your grounding is right before replacing the module.
Always when in for service insist that the codes be given to you. You may do nothing with them but the service tech will think you are watching and checking that what he is trying to sell you on doing makes sense. And posting them enables those on the forums to more accurately focus on what your problem might be.