I believe the wire your looking for is directly under the fog light relay (terminal #5 Dark Green with Light Green stripe) located in the Power Distribution box. You'll need to cut this wire then splice in a new power feed to the side going to the relay socket.
You can either use the park lamps so the fog lights will only come on with your other lamps or you can use a source that get power only with the ignition on. Your choice.
Tape off or put a wire nut on the other end of the wire you cut.
Here's a basic wiring diagram for this mod using the park lamps.