well its like this you run the can over the light parallel to it contouring to the curves. this makes the spray perpendicular to the lens, as it will be sprayed directly at the lense

>>>>>>>>>>>>>>>>>>> = motion of arm
llllllllllllllllllllllllllllllllllllllllllllllllll llllll
llllllllllllllllllllllllllllllllllllllllllllllllll llllll = spray from can
llllllllllllllllllllllllllllllllllllllllllllllllll llllll
_____________________ = lens of tail light

hope that clarifies