[Mr. Adventure]

"This looks like a good place to ask a question about "brake controllers." I have a 2009 Mercury Mariner w/3liter 6cyl motor, 4wd, installed tow package that can evidently tow 3500 lbs but something about the electrical relays limits me to 4pin towing harness which excludes a brake controller. Is a brake controller/electrical brakes essential for even the lightest TM models?"

All trailers over 2000 pounds or so need some kind of brakes in most states. All trailers that come with electric brakes need brake controllers (including TrailManors).

The flat four connector is for brake lights, turn signals, flashers, etc, and the way they come from your vehicle pigtail is just fine for a TrailManor. The wire for the brake controller to the trailer brakes and the wire to provide charge 12v power to the trailer are separate, and in many applications, need to be run separately when they are not provided by the manufacturer. In my trailer connector, the MFR's flat four connector actually plugs into the vehicle-end receptacle to provide the signal, brake, and taillight functions. The other 2 wires had to be run from under the dash (one from the hot side of the ignition switch, and the other from the brake controller).

You cannot infer from the wiring harness the tow rating, whether you have the tow package installed, or whether you have the right hitch or hitch receiver, because many vehicles come with or without these items separately from the tow package (including mine). Any competent RV dealer should be able to help you with this.