EDIT: From what I've read, the '05 Focus did have a CD changer option, which means this module or something similar should plug directly into the back of your stereo. You'll have to find a desirable path to route it out from behind the dashboard, but that's a matter of personal preference. If you don't have a dedicated AUX button, You may have to program the radio to cycle to that option with the press of the CD or RADIO button.
In my experience, AUX tape deck adapters have much better sound quality than the radio transmitters unless you also buy a signal booster, making tape deck adapters a cheaper option as well.
If any of the sister models of your car happened to come with a CD changer, your stock radio probably has the same plug on the back. You'll have to search for your specific model but this is an example of the type of plug you may be able to find.
If all else fails, you can take the car to a car audio specialist (I've had good experiences with 'Car Toys' in the states, but am in no way affiliated with them). I'd tell them you're not interested in spending much money, and ask them if they have any old decks laying around, or any other cheap options before you throw in the towel and go the tape/radio adapter route.