Basically, when you reach at the motel turn to the right side and on the floor there is a pillow. Grab it. When you are by the truck you just need to go and kill the zombie sitting on the floor and, open the car, and get the spark-plug. When you have done that pull that thing by the steering wheel; there should be a long thing that sticks out, pull that, exit the car, and push the car into the zombie by the wall.
Then you should be by the truck again. Stand up and try to break the window again, this time do it with the spark plug. When you start to do it, the man with you will give you and idea and then break it with the remains of the spark-plug.