I installed the DDX919WS in my 2018 BRZ Limited a few months ago.
I couldn't get the factory dual USB to work (the blue & pink plugs under the AUX black box) so I replaced it with the dual unit from Beuler. Works perfectly now, plus it lights up in the same red as the interior lighting.
https://www.ebay.com/itm/BEULER-USBD...-/362568281982
Also, I couldn't get the factory rear view camera to work either. I tried everything, even the PAC Volt 39. So I replaced it with a $40 aftermarket camera from Amazon and now it works perfectly!
I used the custom harness from carboncarsystems.com. I'm not sure if other harnesses from Crutchfield or autoharnesshouse.com would work any better or differently.