Will these mount to the factory fog light openings without modification?

Yes. These are designed for the factory mounting points — no drilling or bracket fabrication needed. Line them up and secure to the existing holes.

Does the switch need to be wired into the factory harness or can I run it standalone?

The switch comes with a harness, but how you integrate it depends on your electrical setup. Some buyers tap into existing fog wiring; others run it as a standalone circuit. Your service manual will show where the factory connector lives if one exists.

Can I replace just one light if it breaks, or do I have to buy the whole kit again?

The kit ships as a pair with one switch. Individual replacement availability depends on stock — contact support with your part number and they can tell you what's available separately.