Getting the radio out of those is a ten minute job in all fairness.
Pull the ashtray out, there's two philips screws hidden behind it, then under the piece of trim that separates the vent controls and the radio, there's another two philips screws. Take all four out and gently pull the grey or black plastic trim towards you, it's easier to work from the bottom up.
If your car has a cig lighter, unplug the two cables (illumination and supply) and gently ease the rest of the fascia out. You'll find that the rear screen heater switch etc are still plugged in, but you should be able to pivot the fascia ontop of the dash and keep it out the way. Saves disconnecting everything really.
There's 4 philips screws holding the radio to the dash, take them out and withdraw it - unplug the cables and put the black plastic dash back on.
Take your PC2-13-4 ISO adaptor, plug it onto the loom and with your new radio, push the cage through the hole in the dash, clip the cables together and push it home. Ensure the aerial is plugged in or you'll find it won't power up. The 'proper' way of fitting it is to run an earth cable (the ISO adaptor will have provision for this) to the metalwork on the dash (basically screw it to the dash using one of the four screws you removed with the radio) but a lot of people just use the aerial earth.
If it's working, screw everything back together and you're done. If not - post up and I'll try and help
(and if yours is not a K11, but one of the first K12s, then disregard all that because it takes a bit more dismantling but it's really only a 30 minute job if you take your time on a K12)