I just went through this procedure recently with my Phaeton. If indeed your rig is on a Freightliner, you will probably find the 6-pin trailer brake connector under the dash on the left side of the steering wheel. After much searching, cussing, and even some bleeding, I finally found mine up above the steering column steel mounting bracket, zip tied to a larger wiring harness.
The way Freightliner explained to me was: Find the wiring harness coming out of the steering column (turn sigs, lights, etc) and follow that harness to the first connector. Then go 4 inches past that connector and the trailer brake connector will be an off shoot of that harness with about 4 - 6 inches of wire before you find the connector. More than likely it will be zip tied and you won't have sufficient room to remove the black connector cover off. So you'll have to cut the zip tie somehow and get the trailer brake connector down to where you can work on it. Once down, remove the small green keeper on the side and then push the locking clip on the side of the cover to remove it and dispose of it. The connector should then be accessible for you to connect to.