You're pretty much right on....but make sure you have the polarity correct. On the bottom of the motor...where one of the wires was attached before you cut it off, there will be a red dot...solder the red wire from the intellifeed on to that.
As far as the rail to gun installation.....it's very easy...just take the sight rail off, take off the left side of the grip...take the trigger frame off of the body, and take off the back plate.....there is a spring in there, so be careful.
Next, just fish the wires down into the gun from the top (its easier to fish them through to the back of the gun where the back plate was, then fish them down into the bottom.)
When you have it down into the bottom of the gun with the other 2 sets of wires, carefully put the frame back on, don't pinch the wires.
On your board, there will be another jack, similiar to the ones that the other 2 harnesses plug into...just plug it in and put your gun back together.
Just make sure that you remember which jacks the wires came out of...it's pretty simple, one won't fit the other, and they are all marked.
"I'm Out Like A Fat Kid In Dogde Ball!"