I think you should have five tables: user, user_car, car, car_user, rental

table user have one to many relationship with table user_car, so you can find how many cars does each user has
table car have one to many relationship with table car_user, so you can find how many users does each car has
table user, rental, car should have one to one relationship among them, so you can find who is having the car now

any comments? good luck