You may enjoy dimming the lights for ambiance in your home, but when your car's lights aren't shining their brightest, this can be inconvenient and dangerous. The first thing you should check when you notice your car's headlights aren't as effective as they used to be is the state of the headlight covers. If the cases are dirty or fogged with moisture, they can restrict the bulbs' ability to illuminate the road.
Keep the lenses clean for brighter lighting
Clean off any dirt or other grime that may be built up on the lenses with a bit of soap and water. If there is moisture on the inside of the lens cover, you should check for cracks or damage around the seal. One way to reverse this problem is to drill small holes in the lenses so the moisture can escape. The process should take a few hours on a dry day, and when it's done you can tape the holes or fill them with silicone glue to prevent water from finding its way in. You'll also need to address any cracks or leaky seals as well.
Get your P's and Q's in order
Before you assume there is a major mechanical or electrical issue behind your lackluster light bulbs, you should inspect the wiring. A loose wire in the lighting system could get jostled while driving and cause the lights to flicker. You may find the fix is as simple as reconnecting a loose wire or repairing a frayed or damaged wire.
Find the root of the problem under the hood
If the lenses are clean and you've noticed the headlights' brightness seems to fluctuate when you accelerate or rev the engine, you may be looking at a more serious problem with the charging system. A faulty alternator or drivebelt could be the cause of the malfunctioning headlights. You should check the belt that drives the alternator for signs of damages - chips, tears and stiffness - and replace it if any of these symptoms are noticed. A loose belt can be tightened. If the belt is not the issue, you will need to use a voltmeter to measure the charging system's power output.
Use the voltmeter to test the voltage of the car battery. You should get a reading between 13.8 and 15 volts. Anything lower could indicate a faulty alternator. You'll also need to inspect the wiring in the charging system for any signs of damage or wear. Start the car and listen closely for a grinding sound coming from the alternator. This could indicate the rotor within is beginning to fail and it is time to replace the alternator.
Give your car a little more juice
Once you've determined the charging system is not to blame for the lack of illumination, you should consider how much power you're using inside the car. When you crank your aftermarket stereo and the lights dim in response, you may be eating up more power than the car can supply. If you have a phone charger plugged in, in addition to an infotainment center, a GPS and a stereo with surround sound and a subwoofer, you might just be using too much electricity. You can either cool it with the tunes and cut back on the in-car electronics, or you can try installing a capacitor. This will not increase the car's power output, but it can provide an extra burst of electricity when the situation calls for it.