I had a 2002 Astra before I bought my new one, and I had an after market headunit wire up no problem- including the steering controls. Has yours got these? You need an adaptor to convert your plugs from your car to ISO connectors plus a 3.5mm jack which you plug into the back of the Alpine headunit. You should have a 2nd set of leads which were supplied with your headunit- plug these into the 2 ISO conectors from the adaptor, and the other end into the back of the headunit.
This link may help you, it describes the kit you need and how to install them (click on the links)
Hope this helps