I have a 2006 Tropical. The wiring harness is tucked up under the left side of the steering column (as you are looking out the front windshield). You may have to do some digging with a flashlight. I got the following adapter from Amazon: Tekonsha 3027-P Trailer Brake Control Harness 2-plug Wiring Adapter - Freightliner.
"If you think it could rattle, it will"
2006 National Tropical LX T350, CAT C7 350HP, MH3000
2014 F150, SMI Air Force One, Blue Ox Avail