What direction does the servo plug go in a Associated XP2G TRS403SSi receiver? I was always told that the signal(white) wire went to the inside, but the plugs don't seem to fit and no the plugs are not keyed.

If you go to page 10 in this manual, you'll see a pic with the servo lead plugged in. Zoom in and you can see the wires. It's black and white, but you can make out which wire is which. You may have to trim the edges of your servo lead plug with a hobby knife if it isn't fitting in.

I don't see the link to the manual that you mentioned.

The good thing with the way things are wired these days is that the red wire is in the middle. With the red wire in the middle, you cannot reverse polarize the servo, it will simply work in one direction, and not work in the other.