Pull the faceplate off, then the trim ring around the faceplate (careful, because it will break). There are 2 flat keys that slide in either side of the radio, in between the radio and the metal sleeve (can, bucket, whatever you want to call it). If you dont have them, go to your local pioneer stereo shop and ask if you can beg/borrow/steal a set. Or pay them the 10 bucks to release it from the dash and you can slide it back in when your done running RCAs.
Keys look like these: