What happens to the forward motion of a passenger in a head-on auto accident?

When the vehicle coems to a sudden stop the passenger's forward motion continues until something stops it - either the seat-belt (the fabric of which stretches slightly to absorb the force, and from which the passenger is thrown back against their seat by a 'recoil'), or the windscreen. If it is the windscreen which stops the passenger's forward motion, the impact usually breaks the windscreen (and some of the passenger's bones) and the passenger may continue their forward motion by going through the glass. Otherwise the impact reaction on the passenger may well be enough to snap their neck and kill them.