If you get locked in the trunk, try to destroy the taillights. As far as I know, they are actually always accessible from the inside of the trunk, often only covered by some plastic or rubber. You can pull out the cables that may only be attached. Or kick the lights vigorously several times.
If you're a little lucky, the police will stop the car because of the broken taillights and flashing lights. Now it should be easy for you to make yourself noticeable.
Or are you sitting in the front passenger seat? Even if your hands are tied, you can do something:
If your kidnapper is driving through a town or on a busy street, try to throw your whole upper body quickly between the driver and the steering wheel and press the horn for as long as possible (usually around the middle of the steering wheel).