||5 pin plug or 7? You'd find a tap for each signal light, one for the brakelight and one for the running lights and then find a good ground (5 pin plug), the wire the trailer to match. This will require a seperate amber signal for turn signals on the trailer.
There are adapter boxes to combine the turn signal circuit with the brake light circuit to maintain 1 light on the back of the trailer, or you can put some heavy duty diodes into the brake light wire to the trailer. Keep in mind that the light will go from flashing to on when you hit the brakes if you use this set up and you might burn bulbs prematurely.
Hope that helsp!