Cadillac 2000 DeVille Owner's Manual page 341

4. Turn on the low beam headlamps and cover the lamp
not being evaluated. The ambient light should allow
the beam cut off to be seen on the wall.
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5. Turn the vertical aiming screw (C) until the
headlamp beam is aimed to the horizontal line. It is
recommended to cover the lamp that is not being
evaluated to better see the beam being aimed.