This page of the trophy guide for Far Cry 6 explains how to unlock the Road Rage trophy. It's quite complicated to achieve - you have to take down a vehicle with a machete while riding a horse. You have to switch from the horse to the enemy vehicle and steal it. We have described the entire process step by step.
Getting a horse
You must start by acquiring any horse. You can e.g. visit any of the Horse Pickups and choose one of the unlocked mounts.
Alternatively, you can jump on a "free" horse or throw a civilian or soldier off their mount.
Attacking and hijacking an enemy vehicle from horseback
Once you get your horse, head out onto the main asphalt roads and look for enemy jeeps.
Contrary to what the trophy description may suggest, you don't have to use the machete yourself. Instead, catch up with the vehicle from the driver's or passenger's side. An icon will tell you that you can now hijack the enemy vehicle - press the right analog stick to do so.
Your character should automatically jump into the cab of the vehicle and kill the driver with the machete. If you performed this attack correctly, the trophy will unlock in a moment.