You seem to be making hard work out of this project.
I certainly am making hard work out of this project, aren't I. Two or three times, I thought I had it solved, only to have it not work and have to go back to the drawing board.
You can drive an open collector transistor or FET from the arduino. Your 50ohms load is connected between your 12 volt supply rail and the collector of the transistor.
I can't do that because I only have one wire that goes to my "load." Not two.
If there's a datasheet for the device it would be really re-assuring to see it - I don't know how such a thing works and what sort of frequencies of PWM are suitable.
There is no datasheet. There is a manual: www.championtrailers.com/BRAKERITEMANUAL.pdf
The wiring diagram is on page 19 of the manual. I called up Titan and they said that PWM will work fine, at frequencies from 200 Hertz to 20 kilohertz.
Also automotive devices often are permanently connected to the chassis/ground and can only be switched on the high-side - if this is so you'll need a PNP or p-channel device to switch and some kind of level-shifting circuit to control this from the Arduino.
I think that is the situation here. The device is wired to ground and I just have one signal wire that I need to work with. I need to source a signal between 0 Volts and 12 Volts using the Arduino, at currents up to about 240 milliamps. At least that is what I have come up with so far.