What you need to do is a bit weird. A more sensible option is probably to UV map them from the top or sides, and then use a simple gradient.
To do what I did, you need to set the camera so that you're facing the headlight like so:
UV map it with Project from View (Bounds):
Under UVs, make sure "snap to pixels" is checked, and move the points until they look something like this:
Then, that is the part of the square flare image that each of those four faces will use.
As always, don't just use things because they're there! Use some common sense! Use a flat gradient or make sure you're using the square flare the way it's meant to be used.