If the fuse doesn't blow until you press the brake pedal, that proves the brake light switch is okay. Even if the switch was shorted, the symptom would be the brake lights stay on all the time. You have something shorted after the brake light switch. That is what we have to find.
Before you do anything else, check if there is a trailer wiring harness added on and if those wires are chewed up.
The first thing to do to narrow down which circuit needs to be diagnosed is to observe there are three separate circuits. There's the left rear brake light, the right rear brake light, and the center high-mount brake light. As luck would have it, the two rear brake lights are the same ones used for the turn signals. If either of those circuits are shorted, the turn signal fuse will blow when that side is activated. See if the signals work on both sides or if one side is dead.
Once the shorted circuit is identified, it is too costly an ineffective to keep throwing new fuses at it. Instead, plug in a pair of universal crimp-type spade terminals in place of the blown fuse, then use a pair of small clip leads to connect them to a small 12-volt bulb. A 3057 brake light bulb works well for this type of circuit. The bulb will limit current to a safe one amp. When the circuit is turned on and the short is present, the bulb will be full brightness and hot, so be careful what it is laying against. Now you can move things around and unplug connectors to see what makes the short go away. When the short is out of the circuit, the bulb will go dim, and you may see the other brake lights turn on very dimly. The first drawing shows using the bulb in place of the fuse.
The second diagram shows the brake light switch. The short has to be someplace after the blue arrow. That circuit goes directly to the center high-mount brake light, but it has to go through the turn signal switch to get to the two rear brake lights. If you follow the white / tan wire to the right and up, it goes to terminal 14 of the turn signal switch. In the third diagram, the blue arrow shows terminal 14 again, and if you follow it around through the switch assembly, it comes out on terminals 15 and 12 when the switch is at rest. Those are the two individual rear brake light circuits. The triangles with "C" and "D" inside indicate the circuits are continued on the next diagram.
Diagram four continues those circuits to the rear brake light bulbs. These are the two circuits where the short has to be, or it is on the circuit going to the center high-mount light shown in the second diagram.
If you haven't identified which of the three circuits has the short, consider unplugging the connector from the back of the turn signal switch. There is usually a bolt to remove, then the plug can be pulled out. That will isolate the two rear brake / turn signal circuits from the fuse or test bulb. If the test bulb is only dim now when you press the brake pedal, the short is not in the circuit. You can put a new fuse in and observe the center brake light works properly.
Let me know what you find up to this point, then we'll figure out where to go next.
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Wednesday, March 17th, 2021 AT 6:23 PM
(Merged)
