The headlights on the Mercury Grand Marquis may become dim, come on intermittently, fail to shut off, or stop working all together. 

 

This vehicle is equipped with a control module for the exterior lights, an automatic headlight switch, sensors to detect when the lights should function, and, of course, relays, fuses, and headlamps. 

Any one of these components may cause the failure, but most often it is diagnosed as a faulty automatic headlamp switch or relay. 

 

In the event that the switch, fuse, and relay are tested and working properly, the vehicle will need diagnostics through professional scanning equipment to understand the source of the issue.