
How can a witness tell that someone in a burning truck is dead rather than
unconscious?


What kind of witnesses?  If we are talking about witnesses who were at
the accident, or were otherwise directly involved (e.g., paramedics,
emergency room doctors, etc.), then they should have been used at the
first trial.  You don't get a new trial because you screwed up and
forgot to call all of your witnesses.

If we are talking about new expert witnesses who will offer new
interpretations of the data, note that the loser can *ALWAYS* find
such witnesses.  If this were grounds for a new trial, then the loser
could *ALWAYS* get a new trial, and keep doing so until the loser
becomes a winner (and then the other side would come up with new
expert witnesses).