Some car audio leads do have the extra wire, which they use to switch a remote amplifier on (via a relay to handle the high currents), but the ones I’ve seen have been too high in capacitance for turntables, and that’s rarely mentioned, so you need to measure them to make sure they are actually low capacitance (150pF or less for a turntable, and preferably below 100pF).
Other than that, it may be best to use low capacitance 2 RCA to 2 RCA leads, and a separate earth wire for the ground – you can always use cable ties or tape to keep the earth wire together with the RCA leads.