I did more research myself - the rogue cable appears to be something to do with the passenger airbag but the full story seems to be:-
Behind the glove box you will find the main wiring harness.
On pre 08/2008 Mondeo's (fitted with BT/VT) there was an 8 pin socket and a plug looped wires to control the microphone wiring (6000/Sony or satnav).
On post 08/2008 Mondeo's (BT/VC now fitted as standard) there is a 22 pin socket and plug with looped wires, this varies the AUX socket wirng for BT/VC only (aux direct to stereo) or BT/VC+USB (aux to BT/VC module, stereo output from BT/VC module to stereo).
Therefore AUX socket in glove box has 3 wires.
If you can find a Ford dealer who knows how to upgrade the software on a BT/VC only module (later 19G488 with usb socket), you would only need to get :-
Cable Assembly 1543691 £20.14 Cable from the BVC to the USB Socket
Control AS 1543450 £3.15 USB Socket
Bezel 1540778 £5.55 Plate to mount USB in glove box
Otherwise you will need a 19C112 module as well.
Unfortunately I cannot post any pictures of the above socket, or it's exact position as my Mondeo was a 12/2007 build without BT/VC.
The date the wiring changed is given as 08/2008 by Ford, but any car built within a month or so of this date should be visualy checked (to confirm wiring)before ordering any parts.