!!As of, October 1, 2020, the game has gone free to play so you don't automatically get everything for free. You have to actually unlock it!!
Backfire is still part of the 2nd challenge "wave" you get (it might be random but it seems to be the same for everyone I've talked to).
So complete your challenges (
on the main menu to view them), complete the first 5 challenges to get a new 5. Continue to do this till you see Backfire (Octane should be your default car, as well as a few others).
After this do the normal "quick" way:
Play - Custom Games - Exhibition
Team Size : 1V1
Bot Difficulty: No Bots
Mutator Settings - Max Score: 1 Goal
and start, score a goal, repeat for the other car.