There usually is no fuse on the trailer - just wiring to the lights themselves. The power is supplied from the tow vehicle and that's where any fuses are - on the tow vehicle. As Jake says, they are wired to the tail light circuit because they should be on whenever the tail lights are on. It's not unusual for most or all of the marker lights to be out because the sockets have become corroded - the lights are often pretty cheap and they commonly leak after awhile, exposed as they are to driven rain and dust. But it could be simply a disconneted wire or corroded splice joint.