When I did the mod, I grabbed a door window switch from a 1995 contour.
When I was installing it, I noticed that when only adding 1 extra wire to the switch, it would not light. So I opened up the switch with some small flat screwdrivers and discovered that the LED used a seperate power and ground all for itself. If you open the switch, you can follow the wires down to the bottom and identify which tab is for power and which for ground.
In my case I had to add not only a connector for power, but one for ground as well. I believe that when you say "3 prongs on the bottom," you're in the same boat as me...the extra bottom prong is power or ground (not sure which one off the top of my head).
It seems ford had 2 different designs for the switch, must have saved $.0001/per unit by eliminating the prong later on.
