Do some brake lights get power from the brake switch through the turn signal switch?

Yes. On many vehicles—especially those that use the rear red bulbs for both brake and turn signals—the brake light power passes from the brake light switch through the turn signal (hazard/multifunction) switch so one side can flash while the other shows steady braking. Vehicles with separate amber rear turn signals typically do not route brake power through the turn signal switch, and the center high-mounted stop lamp (CHMSL) almost always bypasses it.

How different vehicles route brake light power

Combined red stop/turn lamps (common on older U.S. vehicles and some trucks)

In systems where the rear red lamps serve as both brake and turn indicators, the electrical path commonly sends brake switch power through the turn signal/hazard switch to manage the left and right outputs independently. This design lets the switch interrupt brake power on the signaling side so that lamp can flash.

• Typical path: battery/fuse → brake light switch (on pedal) → turn signal/hazard switch → left/right rear stop/turn filaments.
• Function: when signaling a turn with the brakes applied, the switch isolates the flashing side while keeping the opposite side solid.
• Hazards: the same switch handles hazard flashing for both sides.
• CHMSL: the high-mount stop lamp is usually fed directly from the brake switch and does not go through the turn signal switch.

This is why a failed turn signal/multifunction switch can cause loss of one or both rear brake lights while the CHMSL still works.

Separate amber rear turn signals (common on many modern and European/Japanese vehicles)

When the rear turn signals are separate amber bulbs (or LEDs), the brake lights are usually on their own circuit and do not rely on the turn signal switch for power routing. The turn signal switch controls only the turn signal circuits, while the brake lights receive a direct feed from the brake switch or a control module.

Modern vehicles with a Body Control Module (BCM)

In many late‑model cars, the brake pedal switch and turn signal switch send low-current signals to a BCM. The BCM then drives the rear lamps electronically (often LED), deciding whether to flash or hold steady. Functionally, the BCM replaces the old mechanical routing through the column switch, but the effect is the same: combined-lamp vehicles still “merge” stop and turn logic, just via software.

Diagnosis tips for odd brake/turn behavior

When brake lights misbehave, a few simple checks can pinpoint whether the turn signal switch is involved or if the issue lies elsewhere. The following steps prioritize quick, telltale symptoms.

• Check the CHMSL: if it lights but both rear brake lamps don’t, suspect the turn signal/multifunction switch or its connector (combined-lamp systems).
• One rear brake lamp out but the CHMSL and the other side work: suspect turn signal switch contacts or a single-side wiring/bulb issue.
• No brake lights at all, including CHMSL: check brake switch power, fuse, or brake switch alignment/failure.
• On vehicles with separate amber turns: if both red brake lamps are out but amber turns work, focus on the brake switch, fuse, or BCM output (not the turn switch).
• Use a test light or scan tool: verify power at the brake switch and, on BCM vehicles, look for brake input status and command outputs.
• Don’t forget grounds and sockets: corroded grounds or melted lamp sockets can mimic switch failures.

Following this sequence helps separate column-switch faults from power, ground, or module issues, saving time and parts.

Common symptoms and likely fixes

Patterns of failure often point to the same root causes. Use the associations below to guide repairs efficiently.

• CHMSL works; both rear brake lamps don’t: turn signal/multifunction switch or its connector (combined stop/turn systems).
• Only one rear brake lamp out; turn on that side is flaky: worn turn signal switch contacts or a bad bulb/socket.
• All brake lights out: blown fuse, failed brake light switch, misadjusted pedal switch, or BCM fault.
• Intermittent operation when tilting/adjusting the steering column: broken wires or loose connector in the column harness.
• LED tails partially lit or hyperflash: internal LED board failure or BCM detection of a fault; may need load resistors only if retrofitted, not as a band-aid.

Matching the symptom to its typical cause prevents unnecessary parts replacement and narrows testing to the most likely components.

Towing and trailer wiring implications

Tow setups must match the vehicle’s stop/turn arrangement. Mismatched wiring can cause backfeeding, dim lights, or lost brake lamps.

• 4‑flat trailers combine stop/turn: vehicles with separate amber turns need a powered converter to merge signals safely.
• 7‑blade RV connectors can support separate brake and turn circuits; proper adapters are required.
• Faulty trailer wiring can feed back into the vehicle and mimic a bad turn signal switch—disconnect the trailer during diagnosis.

Ensuring the correct converter or adapter preserves normal vehicle light behavior and prevents false failures.

Safety and legal considerations

Brake lights are critical safety equipment and legally required. If rear brake lamps fail but the CHMSL works, drive with caution and repair promptly—other drivers may not see your braking without the lower lamps, especially in poor weather.

Summary

Many vehicles with combined red rear stop/turn lamps route brake power from the brake switch through the turn signal (hazard/multifunction) switch so one side can flash while the other shows steady braking. Vehicles with separate amber rear turn signals generally do not. The CHMSL is typically fed directly from the brake switch. If the CHMSL works but the rear brake lamps don’t, suspect the turn signal switch or its wiring on combined-lamp systems; otherwise focus on the brake switch, fuses, grounds, and, on newer cars, the BCM.

Do brake lights go through the turn signal switch?

Yes. The filament of the brake lights and turn signals are shared. The flasher relay is connected to the big filament on the side you are indicating to turn, which is disconnected by the turn signal switch. Both or one of the brake lights may not work if the turn signal switch malfunctions.

Why do both my tail lights blink when the turn signal is on?

When both turn signals blink simultaneously, it often indicates a bulb or socket mismatch causing a short circuit. Check that all bulbs are the correct type and properly seated. Inspect the sockets for corrosion or damage, which can cause improper grounding.

Will a bad turn signal switch cause no brake lights?

A faulty turn signal switch can disrupt brake light operation because both circuits share wiring inside the switch. Test by activating the turn signals and brake pedal separately; if brake lights fail when pressing the pedal, the switch is likely defective.

Where does the brake light switch get power from?

The circuit is essentially the full run of wires from the battery to the fuse block, from the fuse block to the brake light switch and then from the switch to the brake lights themselves.