[Aussie Ed]

From previous discussions on similar topic, the Series 1 Captiva may have different speaker size/speci depending upon the model variant. (not stated).
Read this back post :- Speaker adapter front and rear

The Forum system believes you may be UK located (you didnt complete your World location in your Sign-Up). If you are UK located it might be worth visiting an automotive Audio store (larger Halfords often have a helpful Audio specialist). Go and pick their brains as to what might be compatible and fixings required.

The following link has helpful info on completing your Sign-Up and how to use the Forum back index discussions.
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Good Luck.