What must the prosecution prove to convict the defendant of the possession charge?The defendant was sitting in the passenger seat of another man's...

What the prosecution has to prove here is that the
defendant had what is called "constructive possession" of the bag of marijuana. 
Possession could also be established if the defendant had actual possession of the
drugs, but this does not appear to be the case here.

In
order to prove constructive possession, the prosecution must prove that the defendant
exercised "dominion and control" over the drugs.  To prove that the defendant did in
fact exercise dominion and control, the prosecution must show
enough of the following types of
evidence:

• That the defendant knew of the
  presence of the illegal drugs.


• That he had access to the
  area where the drugs were.


• That he had used drugs
  recently.


• That drug paraphernalia were
  present.


• That the area where the arrest occurred was one
  that drug users often frequent.

There is no
hard and fast rule for how many of these factors must be found or anything like that. 
Instead, this is more of a situation where each jury or court must decide for itself if
enough of the above factors have been proven.

You can see
the whole opinion of the appellate court in this case at the following link.  I can't
make it show up in the links place below, so you'll have to cut and paste if you want to
use
it.

http://scholar.google.com/scholar_case?case=14067197094672353474&q=state+v.+proctor+901&hl=en&as_sdt=2,48&as_vis=1