On my vintage chassis, with air brakes, the park brake set sensor is on the frame above the rear differential ( drivers side ) . As the picture below .
BUT !!! with the years different in our chassis , I'd say a call the the Freightliner free help line with your chassis serial number is in order.
1-800-385-4357
Ask for an air system diagram, to be e mailed to you. It will give you approx. locations.
One of the switches in the picture is low pressure to the park brake , the other is the set sensor.