What radio do you have? For the JVC KD-NX5000 that came in the Roadster 1.5, the other end of the cable with 30-pin connector is hardwired into an adapter module that connects to the JVC, so just replacing the cable might not be possible. The situation might be different with other radios.
You can get an adapter from Apple to go from the 30-pin connector to a lightning connector. There are two kinds, one with a short cable between the connectors and one with no cable. These adapters allow music to play, but with the JVC it does not deliver charging to the phone. That's because the old connector does 12V Firewire charging while the lightning connector is for 5V USB charging. You can get a
dockStubz+ adapter from CableJive to convert the power.