A dead body should weight down the carpet and keep it from sliding around. Of course the dead body will start rotting and you will need to keep replacing it. You could use a live body, but you have to feed those periodically, lest they turn into a dead body and start rotting. This has the advantage of not being permanent, so if you need to remove the carpet to get at something underneath, you just have to lift the body off of it and you are good to go, unlike if you glue the carpet down. Of course, bodies in trunks can be difficult to explain if you ever get pulled over by the cops.