First thing I would look at is your brake light switch. Make sure that you have full brake pedal return (check the return spring too). The pedal has to hit the plunger on the switch and fully press it into the switch body to open the contacts of the switch. For a couple of bucks pick one up at the local parts store (Autozone, etc) and replace it. It will most likely solve the problem if wiring is not the issue.
