I just bought a radio at a tag sale and i cant seem to get it to work. I got a plug and wired it to my truck, and when i turned on the key, all that would happen is the backlight would come on. That was it. I cant get it to slide open so the screen is visible(Im pretty sure the front were the buttons are slides down and a screen is behind it). Do i need to put a code in to get it to work? Thanks.