On either side of the factory radio case there is a spring loaded tab that locks the radio in place when it gets slid into the hole. To release them, a u shaped wire form tool is inserted in the pair of holes on each end of the head unit face. The end of the wire tool is notched so that after being inserted and compressing the tabs it will pull the unit out. Parts stores may have these. I've had mine for several years now, and don't recall where I got them. You could probably make up a set from coat hanger wire with a little patience[sp?].
