If you can make the possession bar with 6 buttons thats all you need.

To get the exit vehicle to work you can just go into your keybindings and bind a key to do that action. Youll just need to make sure that maccarron isnt using that key for anything else. Best way to do that is go /mac bind select an unbinding icon, bind it to the key you want, then hit esc to clear it. This is what i do. I use numpad + to exit the vehicles.