Even newer trucks without a trailer towing package won't have a 7 pin plug and may come with a four flat as standard.If the trailer side has a five pin connector, it has disc brakes and won't back up without a back-up connection from the truck--a signal to the actuator solenoid to prevent application of the trailer brakes. If the truck is less than 10 years old it should have a 7 pin round connector that you can get an adapter to the five pin flat to fix the problem. Otherwise, you need a connection to the back-up lights to the blue wire on the trailer side.