Check your owners manual. If you don't have one copy & paste this link. http://www.genuineservice.com/genuineservice/en/gloveboxresults
It is for the trailer running lights.
DRL stands for Daytime Running Lamps.
i must recomend that you check th fuse located under the hood that says something bout running lamps
daytime running lamps
42
that is usually same fuse as tail lights
Tail lights and brake lights fuse.
There are about 5 fuses for the trailer harness, Trailer Back up,Trailer left turn, Trailer right turn, Trailer Brake, and Trailer hazards. They are located in the underhood fuse box on the Drivers side of your motor. On my 2003 Silverado, the Trailer left/stop and Trailer right/stop fuses are located in the fuse panel just inside the driver's side door. (Not sure about the trailer brake or hazard fuses.....I wasn't having problems with those :). There is one additional fuse for the trailer harness. The 15A fuse for the trailer running lights is located in the fuse panel under the hood, next to the battery. The fuse is labeled "Trailer Park".
There is a relay for the lamps. It's either shorted out or there may be a bad fuse causing this problem also.
FUSE BOX located above brake pedal driver side.
on a commercial trailer you need a minimum of four tail lamps (two for brake two for signals and all four must work as running lights) red lamps marking the rear corners of the trailer and a minimum of three red identification lamps on trailers over 28 feet in length you need mid markers and mid turn lamps (amber) at the front of each trailer you need amber clearance lamps and you ABS indicator lamp must function also the bumper and the side rails of the trailer require DOT approved reflective tape this is the minimum set up any additional lamps added must be functional once they are added
I have no running lights on my 2006 Peterbilt 387 all of the lights work