Honda Fit (GD): Build a JDM-Style LED Third Brake Light

Raise the hatch and work from the inside. Pop the trim cover away from the lamp with a flat-head screwdriver, then back out the two 10mm bolts securing the assembly to the hatch. Unplug the bulb harness connector and lift the whole unit free. Move to a bench for the rest of the work.

The red lens is retained by four clips around its edge. Gently release each clip with a flat-head screwdriver and lift the lens off. Behind it, press the metal retaining bracket inward at its clip points to free it as well, then pull the original bulb from the socket. You are left with the bare plastic housing and its wiring socket.

Six 5mm LEDs reproduce the factory JDM appearance, though you can run more or arrange them in any pattern. Favor orange-red LEDs over deep red: OEM rear lighting sits near 625nm, orange-red matches that DOT shade, and it transmits better through the existing red lens than a deeper 638nm red. Bare LEDs read closest to the OEM tint; a red lens over them is fine as long as the emitter is in the red range. Around 8000mcd is a reasonable brightness target.

Lay the red lens on a piece of perforated prototyping board and trace its outline. Cut just outside the line, then dry-fit the board into the housing and keep trimming until it seats below the level of the lens clips; if it sits too high, the lens will not clip back down. The board is brittle, so cut in small passes and avoid flexing it.

Mark the center of the board and space the LEDs evenly across it; roughly eight holes apart spreads six across the width cleanly. Push each LED through from the front. Keep polarity consistent, for example bending every positive leg one way and every negative leg the opposite way so you can tell them apart later. For the two outermost LEDs, fold their legs flat toward the center so they clear the back wall of the housing. Anchor each LED with a dab of hot glue on its legs and let it set.

Do not run the LEDs directly off the brake feed with no resistor. Wire them so a current-limiting resistor sets the current for a series string. Measured forward voltage is roughly 2.3v per LED and the feed sits around 12.25v, so a resistor across a series of five LEDs targets about 20mA: [{12.25v-(2.3v*5)}/.02A] = 19.23mA. In practice values in the 180-240ohm range are common (banks of a few LEDs each fed through a 240ohm resistor works well). Plug your exact LED forward voltage and count into an online LED resistor calculator to confirm the value.

Solder the resistor in line with the positive side of its LED string, then join the LED legs into a common positive bus and a common negative bus using short lengths of thin hookup or speaker wire; current draw is low, so light gauge wire is fine. Bring every string together into a single positive and single negative lead. Keep the joints tidy and insulated where leads could touch.

Before closing anything up, touch the two leads to a 9 volt battery to confirm all LEDs light. Remember LEDs conduct in one direction only, so if nothing lights, swap the leads. Fix any dim or dead LED now while the board is still accessible.

Refit the metal bracket onto the plastic housing the way it came off, then feed the two output leads out through the existing wiring-socket hole. The bracket adds thickness top and bottom, so the board tends to hold snug on its own. Seat the leads into the corners of the bulb socket connector. The LEDs stand about 0.5mm too tall for the lens to clip effortlessly, so either rest the lens on the LEDs and tape or glue it in place, or carefully force it back into its four clips for a flush OEM look. Retest after forcing the lens in case a joint shifted.

Mount the assembly back onto the hatch with the two 10mm bolts and reconnect the harness plug. Before you finish, have someone press the brake pedal and confirm the light comes on. If it stays dark, the polarity is reversed at the socket, so swap the two leads and test again. Once it lights with the pedal, refit the interior cover.