Fuses protect individual circuits. Slot no. 14 protects the left side turn and stop signal circuit, No. 15 is for the parking lights, No. 16 is for the reversing light and No. 17 is for the right hand side stop and turn light. If you find that the fuse blows repeatedly, then you have a problem that you need to fix. Check the trailer wiring connector where it connects to the truck and all other exposed wires on the trailer. Look for pinched or pinched wires, insulation damage, or burns, and make any necessary repairs. Check all connections, especially the ground wires on the trailer, to see if they are clean and tight.