JFK's foot. By this point he was lying face down across the back seat.
Really??
Out of interest, could you describe the series of motions JFK went through so that his foot ended up over the side of the limo door.
It is clearly Hill's foot.
Look at the pic David posted. Hill's leg is trailing awkwardly behind him.

Now look at the pic in the OP. Hill's leg is no longer trailing awkwardly behind him on the trunk.

Hill is in the process of trying to move his right leg into a more secure position.
It is his foot hanging over the side of the limo door.