Don't rip the dash apart or trash the unit, these are very easy to remove!!
When new, these were issued with a Sony key, which was essentially a flat bit of bar approximately 1mm thick by approximately 4mm wide.
There are two points in which to insert this bar to release the unit from the cage.
First things first, take the face plate off.
The first slot is approximately 30mm in from the left hand side of the head unit, at the top of the face (ie the underside of the surround that you can't remove).
The second slot is hard against the right hand side of the head unit roughly half way down.
If you don't have the key any thin flat bar can be used - in the past I've used spark plug gappers, hacksaw blades, the lot! Or, if you want to wait, pm me your address I'll post you one out, as long as you promise to send it back!
Have a go with something first, and if you still can't do it, let me know.