All you need to do is to get an aftermarket mounting kit for your vechicle.
Does it have to be a Ford 6-CD, or can it be anything? I personallly like
Alpine or Sony Moble ES... the Alpine unit has a built in 6 disc changer in
the unit itself (you can feed in the discs into the head unit) Plus, it has
IPOD controls, which I personally love... just dock the unit, and it will
control it, and display, etc...
As far as wiring, the best way to do it is to get a female harness, which
will allow you to connect the new units harness to it, and then all you
have to do, is plug in the factory wires... everything will be well
marked, between the female harness and the new units wiring outs The
biggest reason for doing this, in my opinion, besides ease of not having
to splice wires inside the dashboard, is if you ever sell the Truck, you
can just unplug the harness, and re-plug in the stock radio... neat and
clean... works with any new stereo on the market... just be sure to get
the correct year model harnness... GM seems to change it's wiring every 4
years, Ford does too, but not as often I have found.
One big thing I would suggest too... Use wire nuts for every connection!
Don't just use Electrical tape, these can come off, and you will end up
shorting out the stereo or worse. I'd use like a 14 or 12 gauge nut...
1/4 inch bare wire is all you need... Good Luck