Replace the center high-mounted stop lamp (third brake light) assembly on the rear of the cab. This safety-critical procedure requires proper testing to ensure brake signal visibility.

Warnings

Tools required

Parts

• Third brake light assembly (CHMSL) × 1 — Ford FL3Z-13A613-A or equivalent
• Foam seal gasket (if damaged) × 1 — Usually included with lamp assembly

Preparation

1. Park vehicle on level surface and apply parking brake
2. Turn ignition to OFF position and remove key
3. Allow any recent brake light operation to cool (LEDs cool quickly but housing may retain heat)
4. Gather all tools and replacement parts
5. Clean exterior area around third brake light to prevent debris from entering cab during removal

Procedure

1. 1

   Access lamp assembly from interior
   Gently pull down the rear center section of the headliner near the rear window. On F-150 models, the headliner is flexible enough to access the lamp mounting area without complete removal. You should see the back of the third brake light assembly and its wiring connector through the opening.
2. 2

   Disconnect electrical connector
   Locate the electrical connector on the back of the third brake light assembly. Press the locking tab on the connector and pull straight back to disconnect. Note the connector orientation for reassembly. Some connectors may have a secondary lock that must be lifted first.
3. 3

   Remove mounting fasteners
   Using the T15 Torx driver, remove the two mounting screws securing the third brake light assembly to the cab. These screws are accessible from inside the cab. Keep screws in a safe location as they may be reused if in good condition. Support the lamp assembly from outside while removing the final screw to prevent it from falling.
4. 4

   Remove lamp assembly
   Carefully pull the third brake light assembly away from the cab exterior. The assembly may be sealed with a foam gasket that provides light resistance. Work slowly to avoid damaging the paint around the mounting area. Bring the assembly and wiring through the headliner opening.
5. 5

   Inspect mounting area and seal
   Clean the mounting surface on the cab exterior with a shop towel. Remove any old gasket material, dirt, or debris. Inspect for signs of water intrusion, corrosion, or paint damage. If water staining is present, verify the foam seal is included with the new assembly or transfer the old seal if it is in perfect condition.
6. 6

   Prepare new lamp assembly
   Remove the new third brake light assembly from packaging. Verify the foam seal gasket is properly positioned on the back of the new assembly. If the seal is separate, install it into the groove on the back of the lamp housing, ensuring complete coverage around the perimeter. Compare the new assembly to the old one to confirm correct replacement part.
7. 7

   Install new lamp assembly
   Feed the wiring connector through the mounting hole from outside the cab. Align the lamp assembly with the mounting holes, ensuring the foam gasket seats evenly against the cab surface. Press firmly but evenly to compress the seal. Have an assistant hold the assembly in position from outside, or use light hand pressure to keep it seated while you work from inside.
8. 8

   Install mounting fasteners
   From inside the cab, start both T15 Torx mounting screws by hand to ensure proper thread engagement. Tighten screws gradually in an alternating pattern to evenly compress the seal gasket. Tighten to finger-tight plus approximately 1/4 turn. Do not use power tools or overtighten as this will crack the plastic housing. Final torque should be snug but not excessive (approximately 1-2 Nm or hand-tight with a 4-inch driver).
9. 9

   Connect electrical connector
   Align the electrical connector with the socket on the back of the new lamp assembly, matching the orientation noted during removal. Push the connector straight in until you hear or feel a positive click. Verify the locking tab is fully engaged. If equipped with a secondary lock, ensure it is pushed down into the locked position.
10. 10

    Reposition headliner
    Carefully guide the headliner back into its original position. Ensure no wiring is pinched between the headliner and the cab structure. The headliner should seat naturally without forcing.

Reassembly

1. Verify all fasteners are properly tightened and the lamp assembly sits flush against the cab exterior with no gaps
2. Confirm no tools or parts were left in the headliner area
3. Ensure the headliner is properly positioned and not distorted

Verification

• Turn ignition to ON position (engine does not need to run)
• Have an assistant press the brake pedal while you observe all three brake lights from behind the vehicle, OR use a wireless brake light tester
• Verify the new third brake light illuminates brightly and consistently when brakes are applied
• Release brake pedal and verify the third brake light turns completely off with no flickering
• Repeat brake application test 5-10 times to confirm consistent operation
• Check for proper seal by inspecting the exterior perimeter of the lamp assembly - no gaps should be visible
• If equipped with trailer brake controller, test with trailer connected to verify signal integrity
• After first test drive, re-verify third brake light operation before returning vehicle to service
• CRITICAL: Do not operate vehicle on public roads if third brake light does not function properly - this is a federal safety requirement

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