You need to determine whether the brake light is on all the time or it's never on. You can determine this with a test light. Remove the tail light lense and the bulb. With the key on you should have power at one of the contacts in the bulb socket but not the other. If this is the case your brake switches are not connecting power for the brake light or that power is being lost to a bad connection somewhere on the way to the socket. If you have power to both contacts it's likely one of your swithches is stuck closed (likely the rear brake pedal switch as it is adjustable).
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