Saab 9-3 / 9-5 Theft Alarm.  These units contain a battery, and frequently will fail due to circuit board damage from this battery wearing out.  If you are getting the service theft alarm message, it means this battery internal to the unit is dead.  If you have been pressing clear and ignoring this problem for more than about 3-6 months, very likely this battery has leaked acid, destroying the board inside your theft alarm siren (this unit).  Typically this will start causing your alarm to randomly just start going off.  This is a USED unit, that has never been rebuilt.  It MAY work as is.  It may NOT.  I will guarantee that this unit is rebuildable (i.e. it has no board damage, and it will work properly, ONCE you salter in a new battery) , and if it's not, I can send you another one.  If you are getting a "service theft alarm" message, get a good used one to rebuild, and swap out your original unit, you will be all set.