Is the bulb physically broken or just doesn't work?
If it's a bayonet type bulb the spring in the socket could be corroded and binding. Could be a bad ground. Check where the grounding screw goes onto the trailer for corrosion and clean that up.
2015 Seaark 1872 MV CC Etec-90
Two Helix 12 CHIRP SI's, , MinnKota Riptide ST80/i-pilot Link, Bob's Hydraulic
2018 Robalo R180 CC, Yamaha 115
Bitter Gun Owner