Front brake: Underneath the brake assembly, right next to the lever pivot bolt. There's a tab on the lever that depresses a button on it when you don't brake. As soon as you pull on the lever, the button pops out and the light should go on.
First I'd check the two wires connected to the switch, see if the connectors are loose or corroded. If that doesn't work, just remove the single screw that holds the switch. Then you can remove and open it. I've had one stop working once, that was just a matter of taking the parts out and reassembling them correctly. You should be able to see how they should be placed in order to work. When you have the parts out anyway, clean the contact surfaces with fine sand paper and give them a little WD-40 or contact spray. I repaired my front brake switch with just a Gerber multitool
If it still doesn't work, get a new one. It's the exact same part on most Suzukis and it's not too expensive.
Rear brake: Located above the brake pedal, there's a spring (probably covered by a rubber hose) connecting the pedal and switch. Again, the switch is triggered by releasing tension. It could just be a matter of adjustment - try unhooking it and see if it works. If it does, adjust it by moving it with the counter nut on it. If it doesn't, try WD-40 or contact spray. Elsewise, try to take it apart or replace it.
If both switches are out of order, check the bulbs...