Brake lights not working

Without knowing the kind of vehicle you have, just start with basics first. Take a test light, hook to good ground (check light works by other fuse power to light) and check each wire at the brake light switch. Should have power on one of the wires. If you don't then you need to check between fuse and that wire. If you have power at the switch, step on brake and check for power coming out onto the other wire. ( that checks to make sure switch works). If power goes thru then you need to trace that wire toward your brake lights for open or for bad ground. Check for power at the light may have bad ground keeping lights from lighting. This should get you started. This is as much info I can give without knowing more on vehicle.

The replacements switch could be defective (not un common).
Your brake light switch (located under the break pedal) could also be the issue. Have you made sure the bulbs were good. It is uncommon for both to go out at the same time but it is possible (especially if you were recently playing with amny electrical, a short was possible.
It would seem the problem is related to your previous somehow. You most likely have some faulty wiring/electrical component causing problems in areas around it. Take it to an autozone for a free diagnostic if the breake light swtich (under pedal) isnt the issue.
(Excerpt from howstuffworks . com)
So you've tested the brake light fuse and the brake light bulbs and there's still no juice flowing to those locations. In this situation, you'll want to check the brake light switch. It's a mechanism that connects and completes the brake light's circuit in the car's electrical system. It's a very basic two-wire switch: One wire controls the power going in while the other wire controls the power going out.
The switch is located near the brake pedal and it's probably marked. Once more, get out your test light and ground it as you did when you checked the fuses. Place the sensor on just one of the two wires and hold the brake pedal down as you do so. Then test the other wire. If power is connected and the switch is working properly, the test bulbs will illuminate. If it doesn't light up, the brake light switch is faulty and will need to be replaced. If your switch is a more complicated setup, consisting of more than two wires, use the owner's manual to locate the primary "power in" and "power out" wires and test those.
If you do all of these things and still can't get your brake lights to work, your car may have some different systems that need to be checked. For instance, some cars' brake lights and turn indicators are wired together, which means you'll need to inspect that combined system and its fuses. Some Japanese cars feature a dedicated "brake light control module," while cars with an integrated computer system usually offer onboard diagnostic scans to pinpoint any problems.

disconnect turn signal switch wires @ column, put power to wires to brake lights. that cks wiring to lights. next put power to wires through turn signal to ck switch. Power runs from brake light switch to turnsignal switch, to lights.

I would expect to find the problem is the turn signal switch. The third brake light power does not go thru the switch.
You would need to check the circuit in and out of the turn signal switch for power.

It could be the turn signal switch, since the brake wire travels through the turn switch after it leaves the brake switch and then goes to the bulbs. That is how you get a steady brake light on one side, and a blinking light on the turning side. The rear turn signal lights and brake lights share the same filament in the bulbs (and the same wire). The wire on the turning side has to first travel through the flasher relay.
If you have a test light, first check for power out of the brake light switch with the pedal depressed. Then check for power in and power out at the turn switch connector-power in on the brake light wire and power out on either of the wires going to the rear bulbs-if no power out, the turn switch is bad-brake pedal still depressed so you still have the signal.

check the fuses in the fuse panel,if the signal lights are working at the back then the bulbs are good(have seen all tail lights burnt)then you will have to check the brake light switch at the base of the brake pedal.its a 2 wire switch.one wire should have voltage all the time and the other only when the brake is depressed.if the power is not going through the switch then the switch needs replacing.also check for broken wires at the switch also.this should fix the problem.if the third light is not working chances are the bulb needs replacing.

WITH A GOOD FUSE TESTER, TEST ALL THE FUSES FUSEBOX UNDER THE HOOD & INSIDE THE VEHICLE. IF THE FUSES ARE ALL GOOD WITH THE TESTER YOU MUST HAVE A BAD BRAKE LIGHT SWITCH OR A BAD TURN SIGNAL SWITCH. THE BRAKE LIGHT CIRCUIT GOES UP THROUGH THE TURN SIGNAL SWITCH & THEN TO THE BRAKE LIGHTS. FIRST OF ALL IS THE BRAKE LIGHT SWITCH GOOD ? THE ORANGE WIRE FEEDS THE BRAKE LIGHT SWITCH, THE WHITE WIRE PUTS POWER TO THE TURN SIGNAL SWITCH, SO THE LIGHTS CAN BE USED AS TURN LIGHTS OR BRAKE LIGHTS. , PUT A TESTER IN THE WHITE WIRE & PRESS ON THE BRAKE PEDAL. IF THE TEST LIGHT LIGHTS UP, BRAKE LIGHT SWITCH IS GOOD, BY THE WAY THE BRAKE LIGHT FUSE MAY BE LABELED" STOP", THE FUSE LABELED" BRAKE" IS FOR THE ABS SYSTEM. TO CHECK THE TURN SIGNAL SWITCH FOR BRAKE LIGHTS, REMOVE THE PLASTIC COVER FROM UNDER THE STEERING WHEEL, PRESS ON THE BRAKE PEDAL, PUT THE TESTER IN THE SMALL YELLOW WIRE, THEN THE SMALL GREEN WIRE. IF THERE IS NO POWER IN EITHER WIRE, THE TURN SIGNAL SWITCH IS BAD.

the brake light switch powers the wire which goes to the turn signal switch. So I think if the turn signals works, then the turn signal switch is bad. Check for power at the wire going from the brake switch to the turn signal switch. . Since you didnt tell what year or model the truck is, I cant tell you what the wire color is.

Chances are the brake switch has failed. The switch is located at the brake pedal in the front or behind the pedal arm. You will have to be on your back looking in order to see it. There will be a two wire switch. When the brake pedal is depressed the contacts on the switch open (or close depending on how the switch is placed) that in turn illuminates the brake lights. Easy to check with a 12 volt test light. One of the wires at the connector will be the power wire, with the test light grounded, probe one of the wires - the power wire should light the test light up. If neither wire light the test light then you have an open on the power circuit. You will need a wiring diagram to find out which wire is the power wire and trace it back until you find the open. If the switch is getting power and the brake lights are not illuminating,, the switch is bad and will have to be replaced. A fairly inexpensive part and fairly easy to replace. If you have little or no mechanical experience you may need to get a technician to replace it. The replacement switch may come with instructions on how to replace it. Hope this helps.

check behind the brake peadle for a little button that pushes in and out when the peadle is pressed(this is your brake light switch) some cars have more than one of theese. manualy let the button out if the light works the switch is good if it does not disconect the harness from the switch and take a small piece of wire and jump the two prongs on the harness if the brake lights come on then you need a brake light switch. if this doesnt work check for power at the switch. if no power you have a bad fuse if there is power and you already jumped the wires to the switch then you have a broken wire between the switch and the tail lights.