Will these mount at the factory fog light locations without drilling?

Yes — these are designed to use existing factory mounting points on the Prius C. No drilling or cutting required.

Does the switch come wired and ready to plug in?

Switch is included, but wiring harness routing depends on your vehicle's dash setup. Plan on running the switch wire through the steering column area and tapping into the fuse box — standard DIY install with basic hand tools.

Can I use these lights in place of my factory fogs, or do I need both?

These replace factory fogs entirely. If your Prius C came with factory fog lights, this assembly installs into the same location. If it didn't have them, you're adding them from scratch — no conflict with existing wiring.