Where can you find a diagram or instructions that show how to replace a brake light switch in a 2004 Daewoo Leganza?

No direct answer but this may help.

I have changed the brake light switch on my 1998 Leganza Automatic.

The switch is located above the brake/clutch pedal. It is a normally-off push switch which is activated as the brake pedal is depressed. Turn switch through 90 degrees to release it. Unplug cable harness. Replace with new switch and reconnect to socket. It is most likely that you will have to adjust the striking rod so the brake lights respond correctly.(Simply release the locknut, adjust in or out and tighten locknut again.

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My 1999 Leganza switch is in the same place as the previous answer. This switch, when defective, causes symptoms such as the gear shift locking in park.

If the brake switch is becoming defective constantly, you might want to check brake lights - they may be drawing too much power wich might be burning out the switch