I can confirm that holding the remote next to the head doesn't work, but touching chin, temple or the back of the head does extend the range. This works not just with car remotes, but also with garage door openers.
My favorite place (and yes I do this since I saw the Top Gear) is under the chin, pointing the fob/key upwards and opening my mouth. So it doesn't use the skull's cavity but the mouth's (or so I think/believe)
