'Pre-wired' would be a misnomer, since you're doing 'wiring' by installing aftermarket fogs. It probably just has the switch and relay already connected to the wiring harness (or there are connectors). This is versus just buying wire, a relay, a switch, and some spade terminals.
Your car may be 'pre-wired' for fogs, but you can't just pop them in. Most cars have plastic delete panels in the bumper. Hopefully those come out easily and the factory foglight assembly slips right in. They'll need a sturdier mount than the snap-in panels, though, and that may not be provided. What IS there, in most cases, are the harness connectors, which often just dangle with some rubber caps on them or fit into slots on the back of the bumper cover.
But in addition to the probably-pricey foglights from the dealer, you also need a switch. Depending on the car, this may just pop into a delete panel, or you may need to change out the turn signal stalk. Which might make you change out other things on the column, if the proper connections aren't there.
If you're fine with the look of the aftermarkets, just use them. You'll save a lot of money, and if the installer isn't a total fucking retard (50/50 chance there, at best) it should look fairly professional when done.
edit: and use proper goddamn circuit protection. jamming a bare wire behind the blade of a random fuse is not it, nor are hot wires running straight off the battery.