* Wiring Harness: A common culprit is damaged or corroded wiring in the harness that runs to the rear of the car. This harness carries power to the brake lights, the horn, and potentially signals related to the check engine light (though that's less likely to cause *all* those systems to fail). Look for breaks, chafing, or corrosion, particularly where the harness passes through the body of the car (rubber grommets, etc).
* Brake Light Switch: This switch, located near the brake pedal, activates the brake lights when you press the pedal. If it's faulty, it could explain the brake light failure and might even impact other systems indirectly (though unlikely the horn). This is a relatively inexpensive and easy part to replace.
* Multi-Function Switch (Steering Wheel Switch): The horn often shares a circuit with other components within the steering wheel's multi-function switch. A faulty switch could explain both the horn and brake light issues if they share common grounds or pathways.
* Body Control Module (BCM): The BCM acts as a central control unit for many electrical components. A malfunctioning BCM could affect a multitude of functions, including the ones you've described. This is a more expensive and complex repair, usually requiring diagnostic tools.
* Grounding Issues: A poor ground connection somewhere in the electrical system could affect several circuits simultaneously. Check all ground points related to the brake lights, horn, and relevant engine control areas (often found on the chassis or engine block).
What to do now:
1. Thorough Visual Inspection: Carefully examine the wiring harness leading to the rear lights, focusing on areas prone to wear and tear.
2. Check the Brake Light Switch: This is relatively easy to access and test with a multimeter. A simple online search for "2003 Honda Civic brake light switch replacement" will provide visual guides.
3. Test the Horn Circuit: Use a multimeter to check if power is reaching the horn.
4. Obtain a Diagnostic Code Scan: A check engine light, even if not directly related to the other issues, might reveal a code that provides further clues about the underlying problem. An OBD-II scanner is inexpensive and readily available.
Important Note: If you're not comfortable working with car electrical systems, it's best to take your Civic to a qualified mechanic. Improper repairs can lead to further damage or even create safety hazards. The multiple systems affected suggest a serious problem and professional help is advisable.
