I have a question, and it may be silly. But I recently purchased my an sc400. I couldn't get in the trunk because main key didn't work. Long story short, I was able to get into the trunk for the first time and discovered an 12 disc changer in the trunk. The issue that I am having is trying to figure out how it works with my radio, because the is no CD button on it. How do you control it from the radio?