There are locks on both sides of the stock radio. You will need to purchase a set of unlocking tools from Radio Shack. They are perhaps 5 inches long, metal, and slide in the track on each side of the radio. Once they are inserted, you should be able to pull the radio out easily enough. I'd check the radio cables first to make sure you have enough length to pull the radio completely out, otherwise you'll have to disconnect the wiring harness and antenna before unlocking it.
