changing the headunit

o.k people, have a question cause i cant believe what Ford are saying to me....can i or can i not change the headunit and put my aftermarket clarion in??? Ford said if i remove the 3 parts of the standard Cd and radio i will imobilise the car...is this true???? they said it is like the BA falcon...but im sure ive heard of people doing this......Ford i tell you....money makers.
Anyhow if i can, what about the 3 slots? how do over come that, i dont want to cut it really but ford say they dont do anything aftermarket to accomodate a single din unit....so would it look a mess...mitch this is for you id be getting newcastle prosound to do it....but they are saying cant do it at the moment. So is the computor behind the stereo or what????
NOT HAPPY JAN : .....and if i did cut it, how much would it cost for me to rplace it when i come to sell the car and want to pit it back to original.....its just money, money, money all the time.
P.s this site is whay cooler then the old one, i like it