The brake light switch is right above the brake pedal, should have a harness with 3 or 4 wires going into it. When the switches go bad, the brake lights just stay on, as its a normally closed switch.

As for "relays", there are none in the brake light system. There are, however, diode/resistor packs at the tail lights that only trigger the "Brake Lamp" indicator on the dash if a bulb is out.