£110?! That's ridiculous!
I fitted an aux socket on my '07 Focus, which has [url="http://www.caraudio-service.ru/Ford_6000-CD_Radio-Single-CD-6000-CD_5M5T-18C815-FA.jpg"]this[/url] radio.
You need Ford part number 1426121. I got mine from my local Ford dealer for £16.81 although that was only after I saw them selling it at that price on eBay (had been quoted about a fiver more over the phone!).
[url="http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=370335931974"]This[/url] is them and might be the cheapest online.
You will also need four release keys to get the radio out. I got them from [url="http://cgi.ebay.co.uk/PC5-132-Ford-Mondeo-Fiesta-Focus-Stereo-Release-Keys-x4-/150424908188?cmd=ViewItem&pt=LH_DefaultDomain_3&hash=item230605f19c"]this[/url] guy as he was the cheapest I found.
Once you've got everything you just need to drill a hole in the glove box (best to remove it first!) to fit the socket, make sure it's nice and snug. Then use the keys to take the radio out plug one end of the lead into the back of the radio, feed the wire through the dash to the glove box and plug the other end into the socket in the glove box and refit the glove box. Takes about half an hour to do and is pretty simple.