Dimmer switches in cars are very similar to the dimmer switches that might be inside a house. They might control the lights on the dash, overhead lighting in the cabin and other interior lights. Many important dash lighting like the speedometer and warning lights are often controlled through the dimmer switch. Dim lights in the dark can be quite important. Bright lights inside the cabin can create glare on the inside of the glass and make it difficult to see clearly and increase eye strain. Dimmer switches work in two different ways. Older switches work by employing a resistor that decreases the electrical flow to the light bulb and makes the light less. Newer vehicles contain switches that turn the voltage on and off. This is more energy efficient and dims the light. Repairing and replacing dimmer switches requires some electrical know how. Most of the job is just rewiring.