You shouldn't need a manual for that. You might however need a volt / ohm meeter or maybe one of those automotive test lights but I've never used on of those.
A basic electroincs tutorial: To light a lamp, you must pass a current through the lamp filiment. So if you have a battery and a lamp, you must hook up the filiment of the lamp accross the two battery terminals to allow the current flow. This would require two wires, right? Now think about the battery being mounted in your truck. One of the two wires goes inside the cab and is connected to more than one place. One place is the turn signal handle which is a switch. another is the brake and another is the light switch. Each of these switches have the ability to connect another wire to the one from the battery. These other wires are the ones that go to the trailer. They also happen to go to the back of your truck and connect to the tail lights, turn signal and brake lamps of your truck. All of these lamps also need that other wire that goes all the way back to the battery. That wire is called the ground. It is simply the return path to the battery. A typical trailer has two turn signals, a tail light and a brake light. All of these lamps need a ground to work. So that would be 5 wires total required for those lamps. If you only have 4 wires, maybe you don't have something i listed. The trailer lamps get their ground through the trailer frame. Near the front of the trailer there will be a wire attached to the trailer frame with a screw that goes to it's connector that plugs into the trucks connector. This wire is required or the lamps will not light or stay working correctly. Someone might try to rely on the trailer hitch attached to the trailer ball for providing the electrical connection for the ground. Wrong! That's no good. Before you start on the trailer, make sure you have a good grond on the trailer connector. The Trucks ground connector may be attached to the truck frame as I described the trailer's ground. If so that is fine. To really "know", you need a test lamp or a meter. A test lamp could be someone else's trailer with known working lights. Using either a light or a meter you must be able to measure 12 volts between the connector ground pin and one of the other pins used for lights, brake or turn signals. Once all of these signals are working, then proceed to troble shooting the trailer lights. Start by plugging in the trailer lights. Now goto a lamp socket and determine if the 12 volts is getting to the connector or not. Wires don't tend to go bad so, a wire has either been broken because it snagged on something or the lamp is bad or the lamp has a bad connection at the socket. I'd bet on the last one. There you go. A trailer light manual. Good luck.. Walmart sells an entire light kit for about $29. That's two rear lights with all lamps mentioned and 2 side running lights and of course it comes with new wire that includes the trailer connector.