I doubt you will find the connector installed on any freightliner chassis. The assumption is you will be towing a car and the break systems on them are varied and mostly not electric.
The P3 controller only has four wires, +12 VDC, Ground, Break Switch, and the famous blue wire to apply the brakes. This is not a very hard job at all. Here is the wiring diagram for the P3 controller.
Probably the simplest way to wire the brake switch is to add a relay to the brake lights. This relay is the one to use Amazon.com: Car Auto Truck Relay & Socket SPDT 12V 12 Volt DC 40A AMP 5Pin: Car Electronics
Pins 85,86 are the coil. Wire one to ground and the other to the break light wire. Pins 30,87 are the Normally open contacts. Wire one to ground and the other to the P3 brake input. This serves as an inverter and applies ground to the P3 when the brakes are applied.