One of the two wires attached to the adapter that plugs into the brake light switch has come loose. The adapter that the wire is supposed to connect to basically has a small port that the wire is supposed to plug into, but it seems to be clogged with something (or the wire is damaged to the point where it cannot fit).
The connector on the brake light switch has two wires on most of these cars, and it is a common occurrence for one of the wires to break at the connector. The top cover can be taken off and the wire soldered back on, but the wires are already weak, so they will break in a short time anyway. I recommend a replacement connector with wires to splice into the harness, and fix the problem on a more permanent basis. I recommend having a mechanic, such as one from YourMechanic, inspect your brake light wiring, and possibly replace the connector, and replace the brake light switch, so that you will not have anymore problems.
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