The trailer wiring harness is not included with Curt trailer hitch # C11398 for your 2013 Chevy Sonic RS. The reason is because not everyone who installs a hitch is using it for towing or accessories that have lights. By making it separate the hitch price can be lower.
To add a trailer wiring harness you can use # 118640. This will add a 4-Way flat connector to the rear of the vehicle. I have linked a video showing an example installation for you. view full answer...