There is no need to go routing around the back of the dash in order to remove the stereo or even the cage. Both new and old style of car standard stereos require a 2mm hex (Allen) key and a set of 'din' keys, for old models, or 'double din' keys.
On the facia of the stereo there are 4 black grub screws, remove these with your hex key exposing 4 holes. Insert your din keys and push in until they click. Push the exposed ends towards the doors of your car and pull. 2-5 minutes, Job Done
IF YOU DO NOT PUSH THE KEYS APART (towards your doors) THE STEREO GOES
NOWHERE!
The keys can be bought from halfords for a couple of quid although more than likely if you ask politely they'll just pop the stereo out for you.
There are 3 different parts to getting the cage out and you will need only 2 tools. A small flat headed screwdriver and a PH2 screwdriver (phillips/cross head)
1. The arial connector and cable harness are connected to the back of the cage by small clips. Use the flat headed screwdriver to push these in a slide the connectors out the back of the cage.
2. There is a small screw at the back and in the middle of the cage. This is easily removed with your cross headed screwdriver.
3.Around the outside of the cage there are little tabs that have been pushed behind the dash. Use the flatheaded screwdriver to bend these tabs back up and with a little bit of gentle pursuasion the cage will come free. This is the same for both new and old corsa's the only difference is in the new one there is more room to work.
Hope this helps
Oh and always check what adapters you need for you new stereo. For example a pioneer head unit doesnt need a new ISO lead but just to swap the acc and 12v leads (yellow and red) on the existing harness. Best bet is just to ask the fella in Halfords or local stereo shop. And in the new corsa's you'll need a facia adapter (around £10-£15) which you can buy from halfords or places like
www.caraudiosecurity.com