Pontiac 2004 Grand Prix Owner's Manual page 338

3. At the wall or other flat surface, measure from the
ground upward the recorded distance from Step 2
and draw or tape a horizontal line the width of
the vehicle.
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4. Turn on the low-beam headlamps and place a
piece of cardboard or equivalent in front of the
headlamp not being aimed. This should allow only
the beam of light from the headlamp being
aimed to be seen on the flat surface.
Notice: Do not cover a headlamp to improve beam
cut-off when aiming. Covering a headlamp may
cause excessive heat build-up which may cause
damage to the headlamp.