Key on wire for MSD ignition and one wire alt

I am trying to figure out what wire I should use for key one for the MSD ignition. I am using a one wire alt so basically am not using the ballast or regulator. There are some wires going to them but not sure they do anything. I tried using the radio wire but when the key is turned it kills power to that wire. The car does start when jumped. Thanks for any help.