The carpet is held in with little clips that hold it in, and a couple that you unscrew. This article shows how to do with wiring to retrofit cruise control, which involves accessing the computer in the trunk that is behind the carpet, and shows how to remove it.
http://www.pelicanparts.com/techarticles/Boxster_Tech/92-ELEC-Cruise_Control/92-ELEC-Cruise_Control.htm
Steve