As of UE 5.1 input mappings (project settings → Engine → Input) is being deprecated. Enhanced Input is taking its place. Old inputs still works to a degree, but might not by 5.2.
You need to convert to the new system.
Traditionally, you’d put all input nodes and logic (movement/actions etc) in the controller class. Since you are un-possessing/possessing different PAWN classes (character/Vehicle) it’s best to keep logic in the owning class vs bloating the controller.
e.g. All movement/event/action logic for the character would be coded in the BP_ThirdPersonCharacter class. All vehicle controls would be coded in the Vehicle class.
Input Mapping/Action re-usage…
Let’s say you have an Input axis mapping for Forward and Backward character movement. Instead of re-using the same mapping for vehicle accelerate and reverse, you’d create a new mapping/action for those.
What keys you apply to these mappings doesn’t matter. You an re-use keys.
Forward: W, Accelerate: W … What matters is you have to have different input mappings/actions for each.
Eventually you are going to want to have a settings section in your game to allow for custom keybinding. If you re-use the input mappings then you will not be able to have separate keybinds for character vs vehicle movement.
For example say I want to use WASD for character movement and I want to use the arrow keys for vehicle movement. Re-usage of input mappings would force the usage of WASD or arrows on both. They couldn’t be keyed differently.