Cut in the green edges and adjust vertices to round off the top of the light. Chamfer the red edges by about 0.7 or so.
You have to adjust the light so that it curves with the body of the car. Add an "FFD 4x4x4" modifier to the geometry. This creates a lattice around the object. You can use the lattice to adjust the shape of the object. Expand the lattice sub-objects and select "Control Points." Now as you move the control points, the geometry will change shape. Use the control points to adjust the shape of the geometry so that it lies perfectly on the car geometry. The reason I chose to use a lattice for this is because it's a lot easier than than moving vertices and also, if you mess it up or you don't like what it looks like, you can hit the reset button and it will revert the way it was so you can try again.
When you get it looking the way you want it, collapse the stack and then select the polys shown and delete them. Hide the detached polys, leaving just the base for the light.
Select the polys shown in light blue and extrude them inwards. Chamfer the blue edges. Now unhide the geometry you just hid and put a "Shell" modifier on it then chamfer the edges.