You need to find the air pump by following the pneumatic tubes that run from the door locks. In my mkII, it was in the side of the boot, if i remember correctly. the tubes may have become worn, or disconnected from the pump unit. If this isn't the case, it may be damaged somewhere else. I had this problem, and a bit of duct tape fixed the problem. I cant be sure that its the same in the mkIII, but I would bank on this bein the problem. Good luck!
Jabkebab, April 2008