Hi Coy,
Welcome to the Heartland Owners Forum. You'll find a lot of useful information here along with a great bunch of people who are willing to share their knowledge and experience.
I don't recall seeing previous threads on this specific question, but maybe if you're lucky someone else has gone before you. If no one knows how, you may have to be the pioneer and contact the radio's manufacturer for the codes. It's also possible Directv might have the answer on their technical forum.