There is a bracket that bolts to a stud on the rear of the radio chassis. The bottom of the bracket is a "spring clip" that slides onto the radio support. Once you remove the front trim panel, and the front radio bracket, you can reach behind the radio and undo the bolt, but it's a pain to get to. You should also be able to gently move the radio side to side while pulling towards you and slide the clip off the support. The radio is still plugged in to the harness, so don't pull too hard. Once the clip slides off the support, you can easily unplug it. Here is a url to a pic of the rear bracket:
https://secure.cougarpartscatalog.com/19rabrreus.html
https://secure.cougarpartscatalog.com/19rabrreus.html