If you're going to the trouble of putting big lights on the bumper, make it worth it and get some 100w daylighter kc's or some blackout kc's, but don't let a little wiring scare you, its WAY worth it to bump up from the factory wiring. The factory fog wiring is suspect at best, and is surprisingly shabby. Do yourself a favor and for 100 bucks, get some daylighters or low profile daylighters and what everyone is saying is to use relays, and not to insult your intelligence, but i once didn't have any clue how they worked, and i'm not going to assume you have any knowledge just to make sure this is a well informed post. (if the shoe fits, great, otherwise, don't take it personal that i oversimplify)
A typical relay is just a high voltage gate that allows big current through a gate that is controlled by very small current. relays are evident all over your car, for example your starter solenoid is the most common man-application. you have a key in the ignition that sends a small wire's worth of current to the solenoid, where a 4 or 8 gauge wire comes directly from the battery (short wires=less resistance) and the gate is opened by a small wire that completes a circuit that toggles a transformer rated to handle much more juice than that simple ignition wire from the key can.
You're doing effectively the same thing by taking the wire that was going to the fog lamp (which was a relayed wire anyway) and relaying it AGAIN, since the wire is too wimpy to use in a manly application of candlepower magic. (graphic, ey?)
If you look at a relay, (a typical 5 pin aftermarket bosch style) you see little numbers that mean very simple, consistent things. you have 85, 86, 87, 87a, and 30 stamped on the body of the relay. 30 makes a Y with 87 and 87a. 85 and 86 are a circuit that when completed by adding a ground to 86 and a switched 12v microamperage (like the fog 12v provides) causes the 30 to switch from continuity at non-activated circuit with 87a, to continuity with 87. it doesn't matter if you have 12v constant hooked to 85 and you negative switch from the chassis to 86, as long as the circuit is completed, you get the toggling of the relay.
relays also exist that have no 87a, but rather two 87's. these are fine if you have one big wire coming to 30 and two smaller wires going out each 87 to two headlamps...like your application. this keeps the wiring more clean and typically the connectors are more forgiving when you don't try to smash a ton of stranded wire into too little a connector. remember the key is low resistance and big wire, or else you strain the wire, your alternator, and suffer endurance and candlepower.
so all you need to do to make your application work with the factory switch is to take the existing 12v+ that goes to the foggies and put it to the 85 terminal of your relay. put 86 to chassis ground. 30 will be hooked through an inline fused wire to the battery or distribution block fuse, and 87 will go to your new aftermarket lamp bulb positive. the lamp ground will go to chassis.
it is imperative you use the wire that goes to the fog lamp positive AFTER the relay (easy to do, just get the wire from down by the fog lamp bulb) since this is 12v+, as opposed to ford explorer's switch wire which goes to 86 of the factory relays , which is the ground. this won't get you anything but a chassis to chassis loop, and that won't open the wire. if you put the 85 to 12v+ constant, you can go pre-factory relay, but understand that it's all up to you to decide which voltage you work with. You'll get the led green indicator button to light up in the dash for the OEM switch either way, as long as 12v is going to that post-relay wire...so if you want the led green indicator to work, just don't cut the factory relays out completely.
relays are great. intimidating at first, but kinda like jello, once you start there's always room for more.
happy relaying.
John
