2003 Mercury Grand Marquis - Odd light behavior

with headlight switch in off position park lights come on periodically when car is started and dome lights stay on for long periods of time . will go off when driving for some short periods of time and longer periods sometime ! also battery will go dead overnight most of the time but noy allways

The battery goes dead because the lights are staying on all night when you leave the vehicle. your headlight switch is defective and not shutting the lights when it should.

or…There is a non-replaceable relay that sticks and causes this problem. Sometimes you can tap on the module with your foot, located above and to the right of the gas pedal, and it will start working. This is a dealer only part so you need to either get one from a dealer or a salvage yard. This is a very common failure on these cars.

Faulty door switch can cause this sort of problem. Even though door is closed, computer may think door is open so keeps everything powered up & ready to go, thinking driver is about to start the car. When everything in car is turned off, parked, keys removed, but the dome light is staying on longer than it should, try manually pressing the door switch a few times, see if that has any effect. May have to wait some time, say 1/2 hour, after each experiment to see if dome light goes off or not from the pressings. Door switch is usually located in the lock-hardware door-pillar area, visible with the door open.

The lighting control module may be bad.


The light control module is common problem with these cars.