I had this warning recently on my Mk3 2006 2.0TDCI Ghia. Interior sensors located by the front interior overhead light was replaced. I guess though this message could be activated by an untold number of alarm sys faluts. I've only had the car 2 weeks, purchased 2nd hand from a main ford dealer, the fault appeared after a long wet drive to cornwall from surrey....i hope that the fault really was the interior sensor and not related to any water ingress problems i'm reading more and more about.