What should be tested if both the mirror light and heater are not functioning?

When both the mirror light and heater are not functioning, the most logical component to test first is the mirror ground connection. A solid ground connection is essential for any electrical component to operate properly. If the ground connection is compromised or broken, it can prevent the electrical current from completing the circuit, which is necessary for both the mirror light and heater to work. A defective ground connection can lead to various malfunctions since it disrupts the return path for the electric current.

Testing the mirror ground connection involves checking for continuity and ensuring that there is no corrosion or damage. If the ground connection is good, then the next steps would typically include testing other components like the circuit breaker, relays, or wiring, but establishing a proper ground is always a critical first step in diagnosing electrical issues.

In contrast, while testing wire size, circuit breakers, and relay contacts could potentially help in troubleshooting, they would not be the first things to check when the basic symptoms point to a missing connection like ground, which is often the root of the problem in such scenarios.