the spare wheel is held on by a lump of metal (a bracket) that is attached to the vehicle by a cable. On the van I rented, ther was also a security cable attached to the bracket. You loosen the cable, as another responder said, by turning a socket that is behind a hole in the rear bumper. You have to open the rear door to see the hole. On the van I rented there was a vertical bolt that blocked access to the socket. I had to take the bolt out with a large spanner. The socket can be turned with the wheel nut wrench. The flat end, (looks like a giant screwdriver) fits into the socket, but only in one position. Look with a torch to see how the flat end fits into the socket. When the cable is loose enough to pull the wheel from under the van, you can feed the bracket through the hole in the centre of the wheel, and, voila, you have the spare wheel. The bracket may be fixed to the wheel with a plastic wing nut. Put the old wheel back on the bracket, and tighten the cable. Without the wheel in place, the security cable drags on the floor even if you tighten the cable as much as you can. Check tyre pressure on spare wheel before driving off!!
alex, February 2008