After doing a quick google on "kenwood stereo not work in vauxhall zafira" I got a few results on Yahoo! Answers.
The long and short of it is that:
Vauxhall do not use standard DIN connections on their stereos, which is why you can't do a straight forward swap-out. There are however connector adaption kits available but they aren't cheap apparently (haven't researched this - just going on other peoples responses).
Linky ==>
http://answers.yahoo.com/question/in...7125238AACquYW
It states that if you look on vauxhall's website there's a list of stereos that will work...
So up to you...