I got a female plug from Radio Shack to match the male that I put on the end of the power cable. I then connected the red wire to the center post on the female connector. The ground wire from the drill got connected to the outer shell of the connector. The third connector is a normally-closed switch connected to the outer shell. I connected it to the connector that connected to the battery. This way, when the cable is plugged in, it disconnects the ground lead of the battery.
I drilled a hole big enough for the connector in the plastic plate that covered the wires in the drill. I figured that mounting the connector there would a) protect it and b) make it so I would have to remove the battery to hook up the wire. You could just as easily put the connector any where you could find space in the drill.
And that’s it! I now have a 12v drill that I can use on the trail all day if I need to. And it only cost me bout $20. Gotta love it.