AFAIK you'll have to take the HU out, open it up and see what's messed up.
I used to have a guide for removing the radio that I got from the OS but I can't find it now.
Basically you have to remove the knee bolster under the steering wheel, remove the surround around the instrument panel, sometimes remove the centre consol and then you should be able to get the radio out.
It sounds like a big job but it only takes about 15 minutes or so to get all that stuff off.