You have to ask yourself WHY you want to do this:
Do you want to add a new head unit because the existing one sounds cr@p?
Is it because it has no power/no pre-outs?
Is it due to lack of CD player?
Do you just want to be different?
etc, etc.
When I sold my 16v
Ibiza, it had a very nice Pioneer CD tuner in it plus an Alpine V12 amp and 10 inch sub, so obviously I was VERY dissapointed when I heard the stereo in the 20vt when I got it.
Now I have added a 6 disk changer, changed the front speakers to Alpine 6.5" components and added a Sony ISO-amp that just boosts the standard head units output, and added a 10" sub on an Alpine V12 amp.
The system now is very simple, yet sounds very good.
I highly recommend the Sony XM-423SL amp I have because it sits nicely under the dash and takes all the connections from the ISO plug, so it's installed in minutes.
I too was going to go the head unit under sterring wheel option, but now I feel I don't need to.
I hope my rambling has been of use to someone...