Accessories and Chassis Ground
Notice the chassis ground in the illustration above. The vehicle has a large 4 gauge negative cable but also a smaller gauge wire bolted to the chassis. This smaller wire is the return path for the vehicles accessories. The larger wire is for the starter motor. This reminds us that many manufacturers and aftermarket installers use the chassis ground for their horns, windshield wiper motors, and other accessories. It’s important when troubleshooting these components to remember loose mounting may be causing resistance in the circuit.
Most cars have two or more horns. The horn in the illustration above is common. It’s a member of a pair of horns located behind the grill of a passenger car. It has one power wire and is grounded to the radiator support behind the grill by a small bolt. Its operation depends on a good connection to chassis ground that's not corroded or loose. Any looseness will result in resistance in the circuit. Battery voltage can be measured at the plug with a multimeter while someone pushes the horn pad.
This is also true for so many windshield wiper motors. They are often grounded to the body/chassis through their mounting bolts. The bolt is run through a strap and fastened to the chassis by rubber bushings that hold the motor to the firewall. They eventually dry and compress, resulting in a poor ground connection. If there’s intermittent or no operation of this type of wiper motor, look for looseness and corrosion. It just may have resistance is its ground circuit.