Dimming headlights could be caused by a few different factors. One common reason is a weak or failing alternator. The alternator is responsible for charging the car's battery while the engine is running, and if it is not functioning properly, it may not be providing enough power to keep the headlights bright. Another possibility is a loose or corroded battery connection, which could disrupt the flow of power to the headlights. Additionally, old or worn-out headlight bulbs could also cause dimming. Try checking the alternator, battery connections, and headlight bulbs to troubleshoot the issue.