[kipps]

It was evidently sold separately, but KH recommended it for nearly all cases. They say in their literature that it's to be mounted under the hood, on a firewall or fender somewhere. It could be possible someone mounted it out of sight below the dash, but it's supposed to be easily accessible.

Google "Kelsey Hayes resistor" to get a picture of what it looks like.

I have one on order, but it's not arrived yet, so I might be wrong in my description of how it works. The controller gets a battery hot at all times. The controller outputs on two wires. One is intended to operate the trailer brake lights. I'm ignoring that one, and wiring the trailer brake lights in with the vehicle brake light, like all modern vehicles. The other output wire goes directly to the trailer brakes.

This controller cannot differentiate between a light and heavy trailer. If you brake hard with a light trailer, it will lock up before the truck will, and be skidding across the road. To compensate, Kelsey-Hayes offered a variable resistor that mounts between the controller and the trailer brakes.

If you're pulling a light trailer, you'd pop the hood, reposition the "bridge" part of the resistor to an appropriate setting, then proceed like normal. If you were towing a very heavy trailer, you'd move the "bridge" to bypass the resistor, and give full braking voltage.

It's simply an adjustment you make when switching to a different weight trailer. It's not adjusted automatically or while on the go.

The manual that accompanied my new-in-box controller says the resistor can be skipped if you're always towing over a certain amount. It's a better bet to have that resistor though, so you can confidently plug up to an empty car trailer without worrying about over-applying the brakes.

I got a new-in-box 81740 controller for $50-ish shipped from ebay. I ordered a new-in-box resistor for $20 shipped. If I had watched my sales a little better, I probably could have found both sold together for cheaper. The biggest hassle will be getting the appropriate tee fitting for the brake line. This is not included with a NIB controller, since the factory didn't know what truck it was going to be installed on. I haven't pursued finding that yet.