[LIRS6]

I had a Clarion - if it's like mine was, the "sleeve" that your photo shows has tabs on all four sides of it - these tabs were pressed in behind the metal opening of the dash, in order to secure the sleeve should you remove the body of the radio for security reasons.

Feel up immediately behind the dash, find the tabs and bend them back into the sleeve. Then you should be able to remove the sleeve, bearing in mind that wires are likely attached to the back of it, so don't pull too hard....

Jason