If there are only two wires at the holder, cap them. Review the fuses. Many (all?) modern vehicles have trailer brake, tail, and turn wires that are fused separately from the vehicle's lights. The fuses will be on the tow vehicle and they will not be labeled things like "tail lights" since they are part of an auxiliary circuit that powers the trailer. In some cases the circuits may have been added that have fuses that are outside of the normal fuse and power distribution centers. Look above the driver's feet as one possible location.