I have the spoolmate100 from miller that mates to my miller211 welder. it has the main power lead that plugs into the welder (and gas source if you wish) and the little 4 pin plug that plugs to the controls.
i opended up the gun, two of the four pins are the trigger switch, and the other two are power and ground to the drive motor. with two batteries in series, you can pull 24v for welding, and pull 12v off only one battery (at the same time) to run the drive motor. i tapped in spade terminals and re shrinkwrapped the cable that i cut up so i could still plug into my welder. the trigger switch was then wired to give the drive motor 12v. adapted the plug in power for wire with a batter clamp as shown in the pic, and added a ground clamp to the ground. done.
the weld circuit is always (hot) and feed starts when you pull the trigger. there is not feed or heat adjustment, but it burns 1/4-3/8" comfortably just about where i would set it if i had nobs to turn.