you can buy crimp-on insulation-displacing connectors at auto parts stores so you don't have to cut any existing wires to tap in. the radio is usually the easiest place to find the aux line (unless your cigarette lighter is switched by the ignition as well, my car isn't but some cars are) but if you can get your radio out, you can just probe the wires with a multimeter and see which one switches on and off with the ignition, then just tap into it, simple as that. if you can get a wiring manual for your car, you should have no trouble finding a wire to tap into... surely you can find some in the fusebox, which may be even easier to access than the radio, depending on where it is in your car.