The ones that plug into the Focus stereo are very expensive from what i researched a while ago. Im going to go down the route of a new stereo with aux out and sell the focus one for more
This:
http://www.halfords.com/webapp/wcs/s...egoryId_165474
Is the one i installed in a mates Mondeo and was very impressed with the sound. All you need then is a 3.5 to 3.5 connector and £5 usb ciggy power thing from maplins.
The stereo was £50 when we got it so it should come back down again.
BTW i sold a broken 6 stack ford stereo for £90 on Ebay, a working one will fetch more than your forking out.