Accessories and Chassis Ground
Notice the chassis ground in the illustration above. The vehicle has a large four gauge negative cable, but also a smaller gauge wire bolted to the chassis. This smaller wire is the return path for the vehicle's accessories. The larger wire is for the starter motor. Many manufacturers and aftermarket installers use the chassis ground for their horns, windshield wiper motors, and other accessories. It's essential when troubleshooting these components to remember that loose mounting may be causing resistance in the circuit.
Most cars have two or more horns. The pair of horns in the illustration above is behind the grill of a passenger car. It has one power wire, and its ground is a small bolt fastened to the radiator support. 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 chassis ground is prevalent in so many windshield wiper motors. They too are often grounded to the body/chassis through their mounting bolts. Small bolts run through rubber bushings that hold the wiper motor to the firewall. They eventually dry and compress, resulting in a poor ground connection. If there's intermittent or no operation a wiper motor, look for looseness and corrosion.