Horn, Parking_Lights, Head_Lights, Aux_Lights, Radio_Toggle
Open_Door_L, Open_Door_R, Open_window_L, Open_window_R, Hood, Trunk, Lock_Vehicle (for multiplayer)
Turret_left, turret_right, turret_up, turret_down (then you have to have analog inputs. Turret_X, Turret_Y) and a SLOW_MODIFIER(so you can hold and slow down rotation)
FIRE!, Cycle_Weapon, Fire_1, Fire_2, Fire_3, Fire_4, Fire_5
Shift_up, Shift_down, Neutral, Reverse, First, Second, Thrid, Fourth, Fifth, Sixth, LOW_GEAR
Emergency_Brake_Momentary, Emergency_Brake_Toggle, AUX_1, AUX_2, AUX_3, AUX_4, ETC
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Obviously these are many of the things I could see being used but not everything. I have been thinking about my rFactor and Insterstate76 background. Vehicular Homicide is a sort of hobby of mine.
Some of the above actions should be toggle, some momentary and some should require you in real life to HOLD (the Reverse thru Sixth gear for example)
I assume you would be separating Planes and Boats from these sort of vehicles.