If it's all original equipment, the coax jack by the 12v
receptacle (cigarette lighter-looking) is for the antenna on top of your rig... When you are using the air waves and your roof-top antenna, that is where your TV is connected. There should be a little button near that jack...that is the "booster" and it needs to be pushed in (lit) to get a good signal. Note that if your TV is not digital-ready you will need a converter box to receive the digital signal. (If it's a 2004 and it's the original TV, it most likely is NOT digital-ready...we needed one for our 2006 with a Sony.)
When you switch to cable (or satellite), disconnect the cable going to your TV from the antenna input jack and move it to the coax connection labeled Satellite In. That gives your TV access to the incoming cable (or satellite) connection. Depending on what kind of TV you have, you may have to reset your TV to recognize a cable signal in the TV's set-up menu. You don't have to worry about the digital box stuff if you are on a cable signal.