Brake lights not lighting when braking

try changing the brake switch in the floor behind brake pedal.... pedal activates in when used, sending power to brake lights

The third or High Mounted Brake Light runs straight off the brake light switch on the brake pedal arm. You say you tested the "brake unit"? I assume you mean the third brake light assembly, and if you get power there when someone steps on the brake pedal, then the bulbs are bad.

The power flow for the brake lights goes like this: Fuse ->Brake switch -> Turn signal switch -> Rear lights. The only difference is, if you have a 3rd brake light in your rear window, that gets its power directly from the brake switch. It does not go thru the signal switch first. So, if you are having a brake light problem, and your fuses are good, check the 3rd light in the rear window, if it lights up (assuming the bulbs are good, no one ever changes them), then the problem is most likely in the turn signal switch, hope this helps!

have you checked the bulbs?. my next step would be to test for power at the brake light switch and then step on the pedal and see if power flows through on to the other side of connector, if it does then we know theres power and switch is working good, my next move would be to step on the pedal and have someone watch the brake lights and then with pedal pushed turn 4way flasher on and off and see if the lights lite up for brakes and then wiggle the signal light lever and see if the lights come on, if the brake lights come on when any of these steps are done then you have a bad signal switch, if this does nothing then i would look under the dash and locate the harness wiring from signal switch to it junction, this will be a block of wires held together with a center bolt, wiggle wires while on the brake, i have seen the power wire for the brake lights burn inside connecter, and last would be to check the wiring going to lights inside the truck, good day, hope this helps.

lamp failure light on possible because the SECOND filament in your STOP/TAIL Light Bulb is burned out- change one side and check fail lamp again- repeat on other side- fail lamp works off of resistance-burned or blown bulb is only way it lights unless you have a wiring issue

Did you adjust the brake light switch correctly? To adjust the brake light switch, first press the brake pedal down a few inches, then pull the switch plunger rod out of the switch toward the brake pedal arm, as far as you can without using excessive force. Next, return the brake pedal to the pedal stop position against the pedal stop and pull the brake pedal toward the rear to push the brake light switch actuating plunger rod into the switch housing limited by the travel of the brake pedal arm rear travel stop position. This procedure automatically adjusts the brake light switch correctly.
If the brake lights still don't turn off with the brake pedal released, check the brake pedal arm for sticking or binding that is preventing the brake pedal arm from returning fully to the pedal stop when the brake is released.

check and replace head light switch and multi-funtion switch

First off, are the proper bulbs installed? They will have 2 elements inside. If yes, your brake light actuator needs replacing. (switch located under dash, the brake peddal hits it when you step on the brakes)

I had the same problem. They were on when the headlights were on, but turned off instead of brightening when I pressed the brake. I replaced the bulb and now it gets bright when I press the brake instead of turning off (like it should).

Replace the switch on the brake pedal,fit new globes replace fuse.

check the brake light fuse,if not that check the brake light switch located on pedal.