The FMCA forum has a thread on Brake Controllers for the Freightliners. Apparently there is a harness for the controller under the dash but folks had a hard time finding a controller that would work. Some folks said they contacted Freightliner who in turn sent them a harness to work with controllers.
Do a google search on Freightliner Brake Controller and you'll find a ton of info.
US Army (Ret)
2020 Entegra Accolade 37TS
2019 Jeep Grand Cherokee Trailhawk (Toad)
FMCA - F432054