Remove the radio/cassette player from the dashboard and purchase a CD player unit with the same dimensions + fittings at the rear.
Beware: the Security Code will be required to be entered if / when you wish to replace the Radio/cassette unit.
If the fittings are different then adapter connector cables are available.(from Halfords etc)
Worth visiting you local Car Audio Dealer to seek their (free) advice before purchasing anything else.
RegWorts
July 2008