Use the 4-way tester suggested by DuaneQ. That will verify the signals from your tow vehicle are OK. If they are not, you might want to take your vehicle to a serviced center. If they are, then remove the brake/taillight lenses and check that the lamps are good. If not, then replace them. If they are, then use a multimeter to verify 12 volts is present in the lamp socket when the taillights are turn on or the brakes
are pressed. If the other tests indicate the lamps are good, and the proper signals are coming from the tow vehicle, there will probably be no voltage present indicating an internal problem with the trailer wiring.