Ignition box wiring

I have a 68 cuda that has a modified stock harness.Its brand new.It comes pre-wired for dual field alternator and electronic ignition.What I need to know is where I can find a wire that is live with the key on and also live when the key is in the start position.I am not using a ballast resister.I am trying to hook up the on/off wire for a msd box