I actually have a 2020 radio in my 2013 FR-S, but it's the Subaru head unit. that radio has an additional 5 pin connector that works for the back up camera. there is also room to add camera signals to the 28 pin connector.
for the 28 pin connector.
Pin 2 reverse signal
pin 11 backup camera power
pin 12 back up camera signal (+) center of coaxial cable
pin 13 back up camera signal (-) grounded shield of aftermarket cable
most aftermarket camera will take power from the reverse lights, meaning you don't need pin 11. you will also need to run a signal wire for reverse. which also taps off of the reverse lights.
I found a 28 pin connector in a u pull is yard and I de-pinned a few connectors to have pigtails for adding connections.
I used the aftermarket camera below, only problem is it point down too far.
https://www.crutchfield.com/p_249CUL03/Crux-CUL-03.html