You'll be looking for a short on the supply side of the stop lamps. If you have a digital multimeter you can diagnose this pretty easily. Compared to visually inspecting everything. What you'll do is check for voltage at the fuse holder when the fuse is pulled. Whichever reads no voltage is the lighting side, that is what we're going to probe.
First, leave the brake pedal alone and put the DMM in continuity mode, then probe from the side of the fuse holder that read no voltage, to ground. If there is continuity then the short is currently active, you'll have to start shaking the wiring under the dash and see if it stops. That is how you're going to have to track it down. If you can't get it to act up then you're likely looking for something past the brake switch.
If that's the case you'll have to Jam the brake pedal down or have someone step on it while you check out the wiring.
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