I agree with c1apton's assessment. Additionally, get a new battery with higher reserve power (deep cycle), and take long highway drives more often, if you're doing mostly city driving. It sounds like a combination of your alternator having to do extra work since either your battery is always near discharged or can no longer take a full charge, or your alternator is up against too much resistance when trying to push current through your existing wires. I don't think the electronics you have warrant a 2nd battery. And incidentally, you don't just wire a up another battery in parallel- you need an isolator between them.