If it is the car in your sig...a 99 Mustang....and its an aftermarket radio...you will have to get the removal keys of the brand of radio that it is... i.e,.. kenwood radio removal keys, sony radio removal keys, eclipse radios dont use keys, bluapunkt radio removal keys, etc etc etc.
I just noticed its an Alpine you are taking out...if you remove the trim ring you should set a little slot in the side of the cage with some little tabs on it...push the tab until its in the middle of the "slot" on both sides then the radio will come free....i know most older Alpines are like this, im not too familiar with the model number you have so it might still require alpine removal keys....