Good news about this time around it's easier! The trucks are way more flimsy to lose parts. You cannot get it with the Starter trucks as the Training truck has no panels and the Stunt Truck has side skirts that can't be removed in a timely matter. Or at all, I haven't been able to get them off myself.
Same trick as the last one. Choosing a Circuit Race or a Waypoint Race on Easy. Better if it's a max level truck so you have more speed. Hitting poles or environment straight on can break off the bumper, hood and fenders. Doors can come off with enough force from a front end collision or landing on the back end. bed of the truck and the sides come from landing on the back of the truck flat with enough speed. Pure roof lands can also break off the doors. Just get the pieces missing and finish the race with a win. Easy is super easy as the AI isn't too smart.
One thing that can help with air control is changing the Airborne Throttle. In the pause menu, go to Settings, Gameplay Settings and then 2nd option is Airborne Throttle, change to User Input from Auto-Disable. This makes it so your full throttle as you leave a ramp continues and can make backflips and controlling the truck easier after takeoff.