Ebuyer are currently selling Connects2 Aux and ipod adaptors from £37.84 inc VAT. Ideal if you want to keep the orginal sound system. The aux input uses 3.5mm to cd changer head unit input. I have a similar 2 channel rca input instead of 3.5mm, it is a very good audio solution. Worth a look.
Vauxhall Astra SXi Car PC installed.