Engine still runs with key turned off

[ChaseBBC]

So I finally have my truck back together and just installed a painless harness. Everything seams to be working fine except when I turn the key to the off position the engine doesn't kill. Been reading a few articles and some say that the alternator could be back feeding the ignition switch. Does that sound right or has anyone else had or even heard of this problem? I installed a later model alt with the internal regulator to avoid more wiring and a cleaner look under the hood. I don't know if that matters. Thanks in advance.