Mounted in the rear trunk area of all 2015 Nissan 370Z NISMO models is a factory cross brace. The intent of this brace is to counter flex by mounting a ridged brace to key points on the rear of the chassis, helping to make the car more predictable and corner capable at higher speeds. 
This bar is mounted to these two steel brackets sold here, and can only be found on 2015+ NISMO models.
Use the options below to select the correct side for your application.