The Sidestep body parts crate can be found in a ravine to the east of Ember Valley.
Nearing the part will trigger a sonar beep, and will increase in rate as the player nears the part. Collecting the part, once prompted, will deliver it to Rav's garage on Airfield 73.
The player is required to complete the Tyler: Riot Club questline in chapter 3 - City Lights and collect the Sidestep chassis, before they can collect the body parts crate.
Collecting the body parts crate will deliver it to Rav's garage on Airfield 73.