Brake lights fail to get brighter when brakes are pressed.

kentchris85

the cab brake light (center high mount stop light) is on a totally different wiring harness. So, the only thing you know is that this is not your brake light switch. You want to go ahead and check the wiring harness located by the license lamp breakout under your box running along the inside of the frame, measure on there for continuity and check for broken wires and corrosion.

You also need to check inside the "underhood relay box". There should be a relay for the brakelight circuit. You can verify if it is a good relay by swapping it with one of the other 4 pin relays. You could also temporarily place a jumper between terminal 30 and 87 and see if your brake lights come on. If so, you know that the wiring is okay but you are having a relay issue. If not place a test light in terminal 86 and 85 and press the brake pedal. If the atest light comes on then you have power goin to the relay.

The rear brake lights are wired through the multifunction switch. So a bad switch (that is the turn signal switch on your column) can be at fault as well. At the rear the brake lights and turn signals use the same bulb filament, just not at the same time. Check voltage and ground at the rear.