Do the truck's fuses blow when the truck is not hooked to the trailer? If they don't, the fault is with the trailer. You will have to initially start your inspection at the plug, disassembling it and checking the terminals for loose or broken wires. If you have a seven blade plug, the wire you should first be checking, will be terminated in the 9 o'clock position. The wire colours can vary from trailer to trailer, so use the position in the plug to identify it's colour.

The brake/turn wire's identification can be verified by pulling the plug, and whilst using a VOM, check the flashing of the left turn indicator at the truck's socket. Look at the bulb's socket in the trailer for loose/frayed wires or a hot wire that is frayed and rubbing against the frame. If a problem cannot be identified at either end of the trailer, the entire wire run will have to be inspected, and possibly replaced. Some part of the circuit is touching the frame of the trailer.