Manual Control of Electric Brakes

Hello All,

I want to build a manual control for trailer electric brakes for use when moving the trailer around on it's own with no tow vehicle connected. Basically, I want to be able to connect a 12v battery to the trailer 7 pin plug with some sort of manual control in between so that I can vary the braking force from zero to maximum. Ideally an on/off volume control type thing such that the knob clicks to On and then increases braking for from there to max.

Is this as simple as putting an appropriate rheostat (not a voltage divider / potentiometer) in between the battery and the brakes or more complicated? The brakes draw a maximum of about 6 amps at 12 volts DC. I believe brake controllers work by adjusting the voltage rather than the current, but my grasp of such things is quite tenuous. Your answers will be greatly appreciated.