I have heard stories of varying success with ramming small screwdrivers
or a unbent wire coat hanger into the holes and pulling. The correct
tool for the job though, is a 'double DIN head unit removal key' - they
come as a set of two, a pair of prongs that are pushed into the holes,
pressed outwards (towards the doors of the car) and then pulled damn
hard and it brings the stereo unit out. The stereo is slotted into a
cage which is screwed to the back of the dashboard, with the wiring
plug clipped onto the cage. When you pull the stereo out, it leaves the
cage and wiring connector intact - if you need those out as well then
just remove the single screw on the cage, pull out the cage and unclip
the plug.
Tariq