If we ignore the practical impossibility of the situation, your instinct would tell you that the light from your headlamps would be travelling (towards the brick wall that you are about to hit) at twice the speed of light. After all, if you were travelling in the same car at a more reasonable 50mph, and throw a ball forwards from it at 50mph, you would expect the ball - ignoring wind resistance - to travel towards the wall at (50+50) = 100mph.
This is the simple explanation arrived at using Newtonian physics and the combined speed is calculated by the formula (u + v) where u and v are the speeds of the two objects.
Einstein�s physics dictates that nothing (not even light) can travel relative to something else at a speed greater than c, the speed of light. The simple formula for combined speed has to be modified to accommodate this limitation and it becomes:
(u+v) / (1+(uv/c2))
(Where c is the speed of light)
Substituting low values for u and v results in very little difference between this formula and the simple formula. Two trains approaching each other at 100mph each close at 200mph under Newton, and 199.999999999996mph (or thereabouts) under Einstein and the two are close enough for most purposes. However, two objects approaching each other each travelling at 0.75c would exhibit an impossible (according to Einstein) closing speed of 1.5c under Newton, but only 0.96c under Einstein. The formula ensures that relative speeds cannot exceed c.
So the answer to your question is that, provided tou accept Einstein's theories of relativity, the speed of the light from your headlamps would approach anything in its path at 1.0c (the speed of light). This is the result of the Einsteinian formula if you substitute 1.0c for both u and v.