Perhaps you can distinguish for me when to apply the law of conservation of momentum as opposed to Newtons third law?
Conservation of momentum follows directly from Newton's third law if you assume that the time of interaction is the same for both interacting bodies (which it is unless the interacting bodies travel at speeds approaching the speed of light relative to each other). An action is a force for a certain duration. The "opposite reaction" is an opposite force for the same duration. Force x time = impulse = change in momentum so if the two impulses are equal and opposite, they impart equal and opposite momentum so there is no change in total momentum: momentum is conserved.
The bullet has momentum. It strikes the head. The bullet loses momentum. The head gains the same amount of momentum that the bullet lost - momentum has to be conserved (ignoring for the moment that the head is also connected to the body and, through friction, to the car). So the head starts to move forward as the bullet strikes the back of the head and plows through the brain. But then the head explodes sending brain matter in all forward directions. So now the head and expelled contents must, together, have the same momentum that the head had before the explosive exit wound. This means that the head must recoil in the opposite direction to the ejected matter. The forward momentum of the exploding brain matter (and the equal and opposite rearward momentum imparted to the head) is much greater than the forward momentum of the incoming bullet. So in order to conserve momentum, the head has to recoil rearward with more momentum than the incoming bullet. So the recoil from the explosive exit wound completely overcomes the forward momentum imparted by the bullet impact and sends the head backward. This is seen in Chad Zimmerman's turkey shoot video in which he used jacketed bullets fired through pork ribs attached to the back of the turkey:
http://www.dufourlaw.com/JFK/Photos/turkeyribshot1a.mpg