On the Titan that did not come with the factory hitch, there is a 7-pin round connector, taped up under the drivers side of the rear bumper. That is the trailer plug harness. What I recommend is buying a 7-pin socket (you can get them at Wal-Mart, U-Haul, most auto parts stores), the ones made for GM and Ford also fit the Nissan. Then buy a 7-pin to 4-pin/5-pin flat adapter, then you can hook up to any trailer. This is the right way to do it.

Now, if you are really short on cash and are pretty sure you won't ever need to plug into anything but your boat, here are the wire colors going into that harness. Use wire taps to connect your flat plug.
left turn = green/black
right turn = yellow/black
backup lights = yellow/red
tail lights = red
ground = black

You will also need to make sure you have the tow relays, in the fuse box under the hood, behind the battery.

If you go to the parts store they make them ready to plug in you will have to just unplug the tail lights and plug the wire harness inplace of it then itll plug back into the original spot.