First, a vehicle should be a pawn (logically), thus it can be controlled by an AIController.
The AI is a challenge but if you put some effort in it, you can make it.
First, wrap the same commands as the player. The key input should invoke methods, which the AI also does, except it doesn’t make input but decides which function to call. So it can accelerate, brake and steer.
Now first you have to set a target and compute a path with NavMesh Pathfinding or something else.
You can iterate over the path and differentiate the waypoint deviation from the velocity vector and so you get a certain turn slope and can calculate a good amount of steer. While driving check the deviation from the way and correct steering.
That is no full AI, maybe you want to look over some theoretical tutorials or get some literature, but I hope I could help you with my first thoughts.
Greetz