Ford Mustang Owner's Manual page 257

Installing the headlamp bulb
1. With the flat side of the bulb's plastic base
facing upward, insert the glass end of the
bulb into the socket. You may need to turn
the bulb left or right to line up the grooves
in the plastic base with the tabs in the
socket. When the grooves are aligned, push
the bulb into the socket until the plastic base
contacts the rear of the socket.
2. Slip the bulb retaining ring over the plastic
base until it contacts the rear of the socket.
Lock the ring into the socket by rotating it
clockwise until you feel a "stop."
3. Push the electrical connector into the rear of
the plastic base until it snaps, locking it into
position.
4. Turn the headlamps on and make sure that
they work properly. If the headlamp was
correctly aligned before you changed the
bulb, you should not need to align it again.
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