the reality is much simpler
The plug on the back of the audio unit has a blank space where a multi plug would live.
go to your local ford parts dealer and ask them to look on the accessory section of microcat or ecat for the aux in jack kit for fiesta and fusion.
I don't know why it only shows up as being listed for that vehicle, but it'll work with all the new model audio units with the big black plug on the back.
The kit contains a black purpose made 3mm jack plug as fitted to focus and mondeo and a wiring loom approx. 18" long.
Plug the loom's multiplug into the main connector and refit the connector to the audio unit. Now decide where you want the jack plug and drill a hole as shown on the instruction sheet. Fit the plug and have done with it.
Cost of the kit was about 6 quid i think.
Saves all the messing about.....but you'll still have to change tracks on the ipod/mp3 player.