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Yes, you need to of course check the voltage coming into and going out of the brake light switch, if 12 Volts are there going out and coming in while pressing on the brake pedal, your brake lights aren't getting a good ground or there is a break in the circuit between that switch & the lights.
On occasion the tail lights will work with a weak ground circuit where the Brake lights won't. Best way to determine this is to just hook up the 12 volt tester to the Brake light hot wire & there at the bulb ground, then in an area away from the bulb that has a good possibility of contact/ground.
Really that's it, if the 12Volts is getting to the bulb but it isn't lighting, if you aren't getting the voltage there and the brake switch is hot on both sides when pressed, the break in voltage will have to be traced from that switch to the tail light to find the problem.
This is a common GM specific problem. Your turn signal switch contains an elaborate networking device that has to cancel brakelights while running a turn light. Often times the ground within the turn signal switch is lost causing your situation. After investigating such other possibilities as fuses and loose grounds it may become necessary to replace the turn signal switch. Thank you, Dana
Are the rear bulbs OK? Is the ground good at the rear?are the sockets good? Are there tail lights and a license plate light?then the ground is OK. All the things that you say do not work go through the signal light switch....it may need replacing. First make sure the bulbs and fuses are OK and power is getting to the signal light switch and to the brake light switch. Nice car!
Check your fuses of course,you probably did.Listen your not going to believe this,the wire at the brake light switch can be bad.sometines the wire can break inside the vinyl cover just a milameter.use a test light for presence of voltage at the switch and check a few inches back to see it voltage is present. MAKE SURE YOU HAVE VOLTAGE AT THE SWITCH!
Do you have a trailer plug? Look for corrosion around taillight plug under truck in the back, PULL IT APART to see if there is any corrosion. If you find none, than take the taillight housing out and run a external wire from a clean spot on frame to the ground on housing, it could be a ground issue. Did you test light the break light switch power in power out?
Well, the first this to do is get a test light and have someone depress the brake pedal while you check for power at the bulb socket. If you have power, the next step is to check the ground. Just hook your test light to power and see if the light comes on when you touch the ground part of the light socket. In my experience, most light issues are caused by a ground problem. If the ground turns out to be the issue, you can cheat and just run a short jumper ground wire from the light socket to a ground anywhere near the light.
There are a couple of probable reasons why the brake light are not working, they are:
brake switch - underneath the dash. It is activated when the brake pedal is pushed. It is a spring loaded switch and is normally off. If the sprin is defective/missing then the switch will not activate;
brake light fuse;
corroded bulb socket - most brake lights are integrated with the rear park lights. The bulb socket would have 2 contacts, try checking if they are clean and reaches the mate in the end of the bulb;
wrong installation - the bayonet socket of most park/brake lights combo are offset, there is a right and wrong way if installing it into the bulb socket;
loose wire - it would be required to trace the brake wire to where it joins the rear harness;
loose ground - same as above or a workaround is to wire an additional ground wire from the housing of the bulb socket to any screw that is connected to the body ground.
Hope this be of initial help/idea. Pls post back how things turned up or should you need additional information.