You cant,there is a bypass code available from the dealer so you can bypass the immobiliser (£10-20),you use the central locking button to do this on the car (dont know about the van). This will stop your remote central locking working and of course you will have no immobiliser ,and you have to do it everytime you get in!!. Other than that look on ebay,there is a company that will sell or fit you a permanent bypass for around £90,again you will lose the immobiliser feature,hope that helps.
Deano, January 2007