This is what it looks like... And it ain't cheap.
http://www.rockauto.com/catalog/raframecatalog.php?carcode=1417913&parttype=7192&a=FRc1417913k935268
This one is for a 2003. The one for a 1999 looks similar, if not identical.
Download this file and go to page 24- D10. Items 4 and 11.
http://www.cannell.co.uk/Boxster_Manuals/986%20Group%200a%20Diagnosis%20To%2045.pdf
They are a little tricky to get to.
Bank 1 or 2? Bank 1 is a little easier than Bank 2.
Hope this helps. Good luck.
mike