There's a pdf knocking about somewhere for this and I've got some pics so you can see where to cut the slot.
And stop the radio coming on by itself
And you can just hack up the original box to make room
It's a bit of a job but if you take your time you should be fine.
Also check out the euro fascia, they only got the same as ours in their sports versions but their other cordies and ibizas all took a standard stereo fitment. Dave seres can get them but I can't remember the price, you'd also need the manual heater box from a polo or something for the manual heater controls.