I don't know if this will help you as your vehicle is older, but we've just had a problem with a four month old Ford transit that we hired. The two front doors could be opened manually with the key on the driver's side, but this would not release the doors to the rear sealed unit. Finally, we managed to find someone who gave us two tips. First of all, as the key charges off the battery, try putting the key in the ignition, and turn it to engage the battery. Leave for a while, then remove the key and tap it. If this doesn't work, try inserting the key in the door keyhole, and turn it three times to the open position. This should re-set the central locking system. If neither of these things work, there is something wrong with the key.
Annabelle Johnson, April 2007
