Do these plug into the factory wiring harness or do I need to splice wires?

These come with the integrated switch, so installation is straightforward at the fog light housing. Verify compatibility with your factory harness connectors before ordering—contact your dealer with your VIN if you're unsure about your wiring setup.

Can I run these while the headlights are on, or do they work independently?

The dedicated switch gives you independent control—you can run the fogs separately or together with your headlights. That's the whole point of the integrated switch instead of relying on automatic daytime running light logic.

Are the LED bulbs replaceable, or is it the whole assembly if one burns out?

The bulbs are integrated into these LED units, so replacement means the full assembly. LED lifespan is rated far higher than halogen, so you're looking at years before that becomes a concern on normal driving.