OK. So probably a common problem, but I thought I'd chuck my experience out there.
2007 X-Type estate with water ingress in the rear, when left unresolved, causes damage to the Rear Parking Aid Module, located in the spare tyre well, such that the rear parking sensors stop working.
After checking fuses first (of course) removed the module (three cable connectors and two nuts) and opened up the unit (fairly easy to prise the board out of the housing). Obvious corrosion all over the lower edge of the board and lower pins (lower edge when in fitted position).
Unit was clearly knackered. Mine was model 4X43-15K866-AC (hardware 02, software 03, code 01).
The only bloke on eBay that had the same model wanted £105 (~$155) for a used one and wasn't taking a good, old-fashioned best offer of £90.
So, I took a chance and bought a used later version module 4X43-15K866-AE (hardware 02, software 04, code 01) and bartered the owner down to £80 (~$118).
Today, I fitted the replacement module and all is well.
In my case, I also relocated the replacement module into the polystyrene side tray, next to the spare tyre well, just so that I know it will not suffer from any future water ingress.
I've attached two pictures to this post (I hope) which show both the old and new modules in place, with the new one cabled.
They show how I cut a recess out of the polystyrene tray to accept the new unit and held it in place with a simple roof bolt.
I guess the two key things to consider are; always move up a model, and always ensure the hardware version is the same (so that you know the connectors will fit).