It's a fuel pump switch, it slots in were the fuel pump relay goes so you can turn the pump on and off, it's a great help when you're trying to diagnose a problem.
To make one, get an old fuel pump relay, carefully remove the cover, and gut the inside of the relay so all that's left is the base with these 3 terminals.
Then, solder wires to the terminals and to the switch, if you're using a lighted switch like mine, the wire from terminal 31 goes to ground on the switch, from terminal 30 to power, and terminal 87 to accessory. The light on the switch serves to tell you if the circuit has power. Then drill a hole in the cover for the switch and put it all together.
Modified by ABA Scirocco at 3:53 PM 4-23-2010