You can make a removal tool with a wire hanger, just make them into a U Shape and insert into the slots, insert them as far as you can you will feel it come loose and forward. also try the instructions for removal at crutchfield.com or try this
http://installdr.com/InstallDocs/FLM/PDF/356041.pdf
Also i reccomend getting the wiring harnesses so you can connect the new stereo to the existing car connections.