I have never used one but JC Whitney used to sell one.
Frankly I'm not sure I'd worry about it.. Just use the turn signals.
In fact.... A suggestion:
Take a partner and go out to the car, Now with no key in the ignition (Switch in off or locked position) have your partner step on the brakes... See if the brake lights come on.
Option 2: Do this at night and you need only look in the rear view mirror no need for a partner.
Odds are applying the vehicle's service brakes will turn on the brake lights.
now, use the standard diode kit to operate the turn signals (They will double as stop lights too.
When your aux braking system (NOTE: This will not work if you use the M&G system but it will with most all the others out there like US-Gear, Air Force one, Invisible Brake, or any of the box in the driver's seat systems) The application of the towed brakes will bring the stop lamps on.
No need for a converter at all.
I recommend either US gear or Invisible Brake for this type of install however (They both keep towed battery charged)
Home is where I park it!