Hi Matt, two options. (Cheapest and simplist of the two first) *Try getting a new key cut. With age on those ford keys the profiles wear down so much to the point that the lock doesn't recognise the key anymore. Get the key done at a Fords parts dept. They will cut the key just by looking at it and counting the sequence of the original profiles. Anyone who tries cutting a new key by replicating your old one in a machine will only get an exact copy of the old worn one.
**You need a new lock barrell > (tumbler kit) for the door. These are available from any Ford parts dept, just quote your reg and they can help...from memory they only cost about £25.00. The only problem is you'll have to have the barrell calibrated to your actual key by a specialist locksmith (Yell.com in your area for a locksmith) which depending on where you live should only cost about £50.00
You could always try a re-tumble the lock yourself by putting the new one together in a reverse sequence of dismantling your old one, but this requires a steady hand, a pen & paper to write down the sequence as you remove the tumblers otherwise you will forget, and alot of patience as there is about 50 little bits that all go together to make your lock...just think about the inside of a watch or anything you ever took apart as a kid, how'd all that fit in there question comes to mind?
Good luck!!!
Aaron
January 2008