Before you do the following, carefully look inside the trunk to make sure that a harness plug for the brake light wiring is not disconnected.
It is relatively easy to precisely find the cause of this problem, without wasting time and money on guesswork and unnecessary parts. But it does require a 12 volt test light, a 10 foot length of 16 gauge stranded wire with an alligator clip on each end, 2 additional alligator clips (only required if the test light does not come with alligator clips on its leads), a straight pin or hat pin, some patience, great care not to accidentally short any wires to ground, and the willingness to follow instructions: You can buy 12 volt test lights, wire, and alligator clips at auto supply, electronic supply, and hardware stores. Straight pins or hat pins can be bought in the houseware or sewing section of supermarkets or department stores. These parts are all inexpensive.
The power for the brake lights begins at the battery "+" terminal; goes through the wiring harness, through the firewall to the fuse block; then goes through the brake light fuse and to the brake light switch, then goes through the brake light switch, and through the harness to the light bulbs at the back of the car; then goes through the bulb filament to the light housing; then goes from the light housing through a ground wire to a bolt in the trunk; then goes through the car body to the battery ground cable; and completes the circuit by going up the ground cable to the battery "-" terminal.
Somewhere along this route, the power is being interrupted. It may be at a bad connection, a defective part, or a broken wire. All you have to do to find the problem is to ground one end of the long wire; attach the other end of the long wire to one wire from the test light, and touch the other wire from the test light to various points along the line; to check for voltage. Start at the battery "+" terminal, and continue through the whole list in the previous paragraph. You always need to touch metal parts or wires for these tests. If you want to check a wire which is covered with rubber or plastic insulation, you can clip the test light wire to a pin, and use the pin to penetrate through the insulation, in order to touch the metal conductor inside the wire. When you reach a point where the test light does not glow; you have gone past the point where the power has been blocked. BE SURE THE TWO TEST LIGHT WIRES DO NOT TOUCH EACH OTHER, AND THAT ANY ALLIGATOR CLIPS OR BARE WIRES DO NOT TOUCH OTHER METAL OBJECTS BESIDES THE ONES YOU ARE TESTING. COVER THE CLIPS WITH ELECTRICAL TAPE, IF NEEDED, TO PREVENT THEM FROM TOUCHING A NEARBY HOT OR GROUNDED OBJECT. If the two test light wires touch each other while one is connected to a hot wire; you can get a nasty short, and probably blow a fuse or burn out a fusible link; so be VERY CAREFUL ABOUT THIS.
The test light has two wires on it. If you touch one wire to one of the battery posts, and touch the other wire to the other battery post at the same time (use the 10 foot long wire as an extension if needed), the light should glow. That indicates there is power between those two points. Now clip one end of the 10 foot long wire to an engine bolt. Any bolt you use for these tests must be clean, and not have paint on it. Sometimes you have to scratch or file the surface of a bolt to get a good contact. Attach the other end of the 10 foot wire to one of the test light wires. Connect the other test light wire to the clamp of the "+" battery cable. The light should glow. This indicates that the alligator clips are properly attached to the 10 foot wire, and the battery ground cable is working. Now move the end of the long wire from the engine bolt to a bolt on the firewall, and then to a bolt in a fender. The light should glow at each of those points. That indicates the battery is grounded to both the engine, the firewall, and the fender. If you touch something that is not made of metal (such as the windshield), or is not grounded to the battery, the light will not glow.
Now find a solidly grounded object inside the passenger compartment. One good choice is the bolts which hold the steering column clamped to the dashboard. Another would be the screws which hold the trim or striker plate to the door jamb. But you can test your own ground points by bringing the end of the long wire into the passenger compartment, and touching it to places which you think might be well grounded. If the test light glows when you touch a given point; you have a good ground there.
Once you have a good ground point inside the passenger compartment; bring the test light and the long wire into the passenger compartment; ground one end of the long wire to the ground point, connect the other end of the long wire to one of the test light leads, and touch the remaining test light lead to whatever you want to check for power inside the car. You can check this setup by turning the ignition key to the position where the dashboard warning lights come on, and touching the free test light wire to the terminals that a fuse plugs into in the fuse box (you'll probably have to remove a fuse in order to access the fuse box terminals for that fuse). The light should glow when touched to at least one of the terminals for the brake light fuse. If it also glows when tested while the key is not turned on; then you can leave the key off while you are doing the rest of your testing.
Now unplug the brake light switch, and use the pin or the test light lead to touch each of the two terminals in the plug from the wiring harness (not from the switch). You should get power on one of those terminals. If you don't; be sure the fuse is in place. If the fuse is in place, try another fuse in that position. If you now get power at the plug, the first fuse was probably blown. If there is a short in the brakelight wiring, it will blow a new fuse as soon as you step on the brake pedal. But if you still don't have power at the plug; then the fuse block or the harness between the fuse block and the brake light switch may be defective. You can trace the color of the harness wire back to the fuse block, and probe it with the pin as close to the block as you can get. If you still don't get power, and you're sure you have the right wire; the fuse block is probably bad.
If you have power at the plug for the brakelight switch, then reconnect the plug to the switch, and connect the pin to the harness wire which did not previously have power to it. Then step on the brake pedal; there should now be power at the second harness wire. If there isn't, then the brakelight switch is either defective or out of adjustment. You can confirm this by unplugging the plug, and connecting the test light across the two harness terminals. If all the other parts except the brakelight switch are in place, and the rest of the circuit is good, then the test light will glow. If the light does not glow, and it is receiving power from the plug; then there is an open circuit somewhere beyond that point.
Once you understand how to do this testing, you can use this procedure to go through the rest of the circuit on your own. Once you go past the bulbs and reach the ground side of the light housing; disconnect the ground wire from the body bolt, connect that wire to one side of the test light, and see if the test light glows when the free end of the test light is grounded. If it doesn't glow; run the long wire outside the car all the way back to the battery "-" ground terminal, and see if the light now glows when it is connected to the ground wire from the light housing. If it now glows, the connections between the body panels going from the trunk to the battery are too corroded. In that case, you'll have to permanently run a dedicated ground wire through the car body and back to the battery ground cable. Sometimes when the light housings are installed onto a painted fender, they do not make a good enough ground connection. In those cases; the problem can be fixed by scraping paint off the body where the light housing contacts it.
I hope this helps!!!
Joel
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