check the obvious. LIGHT BULB!
Thread states (Customer states, for those in the auto service industry) that the brake light comes on when the rear brake lever is pressed. That pretty much rules out light bulb. But, normally, that would be the first thing to check. Only a few possibilities left: 1. A faulty switch in the front brake switch in the master cylinder, 2. A loose or faulty connection at this switch, 3. An open circuit in the harness after the front brake switch, 4. A fault (open circuit) in the fuse block in the circuit that controls the brake light (power from the front brake switch).
So, if the connector is plugged in properly at the front master cylinder, start tracing wires to see if something frayed. After that, you're looking at getting someone with a DMM or Power Probe to do some electrical diag for you.