Not so straightfoward as as toureman says. I have a Meriva, and like a lot of modern cars, the audio side of fittings is size specific. You should be able to get the correct fitting from a Vauxhall dealer, but it will cost you a bit.
You could always buy a fairly decent CD Walkman and connect this to your existing radio/cassette via a cassette adaptor.
i have a meriva life 04 plate and it only had casette/radio in i bought cd/radio off ebay which is appropriate for any vauxhall with the code for about 60 quid and its music to my ears