She's not saying that she's some guy's girlfriend. She's telling her friends that she feels the need to hook up with a guy -- any guy. 'Cause when the cars and the signs and the streetlights light up the town (i.e. - nighttime), she gets horny.Why would the girl who is "somebody's baby," refer to herself as somebody's baby - and in the third person?
And what's that last line thrown in there for? (It doesn't relate to the next line either.)