Changing cameras and controller in game

Hello All, So as I'm making a first person shooter game I have my controller but to make it a bit for fun I added cars around the scene, I want to make it that if a player finds the car it is able to change controllers, so it can control the car. So if it comes in the distance of 3f it can click 'g' and it can control the car. May I have some assistance on how to do this. Cheers in Advance