What does the yellow and black wire that says battery go to?

Yellow goes to a 12 volt power source that never goes off no matter what position you have the key in. Black wire is ground and that can be tied into the frame of the car or another ground wire, you can find out what all wires do by using a test light. Clip one end to a clean ground and pone a wire, if it's +12 volt, it'll light up. Turn the key to find out what the wire is and once you find one that stays on no matter what, you have found the 12 volt batter wire that yellow goes to and if you can't find one there, you're going to have to run a small wire from the battery to the radio ( the radio should have an inline fuse, if not, put one on it if you have to wire it to the battery directly). To fine a ground wire you must first find a 12 volt+ wire, clip one end to the + wire and start poking other wires to find a ground wire. (you only need to know this for older care that nobody makes a wiring kit for.) Some places sell plugs that convert factory wire harnesses to after market configuration and that all you'd do is color match the wires. I hope this helps!