Radio keys won't work on some models.. you need to remove the ashtray surround and the heater control cover. It 5 screws and takes about 20 minutes. Some Previa's have the radio screwed in so these need to be unscrewed. The wiring is easy if you can work out what each one is for..
http://www.estimaowners.com/community/f166/fitting-cd-previa-17288/
Max
August 2009