Oh. There is no built in rca in stock head unit for 2013 cx5 sport. Yours is 2013 cx5 sport? Now I'm curious to how yours was wired there. I had to buy that small rca adapter to plug in the empty hole at the back.This is what I used but it’s not available anymore.
https://www.amazon.com/dp/ (commissions earned)
I don’t remember having to use an adapter for plugging into the head unit but I could be wrong. I think there was an RCA input on the back of the head unit and that was it.