2000 GMC Jimmy reinstalling the dash and radio

I just changed out my heater core/ While replacing the dash and radio, I seem to be missing a radio cable. There are female plugs on the back of the radio and I can only find 1 male plug for it. Everythingelse in present for the rest of the dasn connections. A friend helped me remove the dash and unpluged the radio, so I didn't see what was connected at the beginning. I plug is a 4 gang plug and the other is a 9 pin. And thoughts?

Well, I put it together with the 9 pin empty and everything works just fine. I guess it's for add-on's.
Thanks anyway.