Yeah, you basically need to cut the plastic strips or whatever they're called out, then fit that fascia plate adaptor, then a normal stereo should slot right in. Hassle, but it is what makes modding fiestas fun
I've spent a lot of money/time upgrading my whole audio system, it sounds amazing, and was totally worth the hobby! Be careful though, its addictive!
In regards to the sub, again with the fiesta it is difficult... there is no obvious hole for the wire. What I did was use a knife to cut a very small hole (just enough to fit the sub wire coming from the boot) in a rubber casing for the fiesta's own wires, which you will find under the glovebox. poke the wire through and it should come out under the bonnet, so you can get it to the battery. Hope this helped!