First off, clean the connector and check the wiring on the switch pigtail. It's exposed, and might get damaged.
Got a multimeter?
Unhook the switch from the wiring harness and measure it's resistance. It should be high (more than 10K ohms) when the brake is off, low (less than 2 ohms) when it's on. You may need a fair bit of brake pressure to turn the switch on.
Unhook the switch from the wiring harness. Turn the ignition on. Short circuit the two pins in the wiring harness connector to each other with a bent paperclip or something. The brake light should light. If so, the problem is not the harness.
Now you know if the problem is in your switch or the wiring harness.
Bad switch? Check the pigtail again, then try a new switch.
Bad harness? Now you really want a multimeter. Turn on the ignition. Measure the following voltages relative to battery ground:
Yellow/blue wire at connector +9V or more is good
Green/white wire at connector, front brake off +2V or less is good
Green/White wire at connector, front brake on +9V or more is good
Turn off the ignition, and measure the resistance from the green/white wire at the connector to battery ground. Less than 6 ohms is good.