5.1 uses enhanced input. You have to convert over to it.
Put all your control/input logic for the vehicle in the vehicle class.
Put all your control/input logic for the character in the character class.
Put menu/settings and other universal input logic in the controller.
Do not reuse inputs.
There should be a dedicated enhanced input forward for character and a different one for the vehicle.
e.g.
Player movement: Forward, Backward, strafe r, strafe l etc…
Vehicle movement: Accelerate, Reverse, Brake, turn etc.
You can reuse KEYS, just not input actions.