Welcome to the forum! More detail is needed. There are many external light bulbs on a trailer and we don't know which one you are referring to. Pictures are worth 156 words.
2006 23B Hybrid with 10k round bar WDH
2011 F150 4x4 SCREW Ecoboost, Max Tow, Integrated TBC, 3.73 LS axle, Firestone Ride Rite airbags.