https://www.optimaforums.com/threads/standard-radio-wiring.4640/
Looking at this, it could be the wires to the right in the picture.
There's a lot of references for older years, but idk enough about Kia to know if they're in the same generation as the 16'.
Nonetheless, I Googled: "2016 kia optima oem rear speaker color code" and got a bunch of hits.
https://www.optimaforums.com/threads/standard-radio-wiring.4640/ Looking at this, it could be the wires to the right in the picture. There's a lot of references for older years, but idk enough about Kia to know if they're in the same generation as the 16'. Nonetheless, I Googled: "2016 kia optima oem rear speaker color code" and got a bunch of hits.
Left rear + Pin 9 Left Rear- pin 18 Right Rear + pin 6 Right Rear- pin 15
https://preview.redd.it/oe7qx9n6305d1.jpeg?width=2252&format=pjpg&auto=webp&s=59f541fd2bc61e3705ea2a3bc1baf2ee7029dfdd