Do these require splicing into factory wiring, or is there a harness?

They come with a wiring harness that taps into your electrical system. You'll need to identify the appropriate power and ground points under the dash or behind the bumper—exact routing depends on your truck's layout.

Will these bolt straight to the bumper, or do I need to drill new holes?

They mount at factory fog light locations, so no drilling needed if your truck has the OEM cutouts. If your bumper doesn't have pre-drilled holes, you'll need to mark and drill before installation.

Can I run these with my headlights on, or should they be independent?

The included switch gives you independent control—flip it on or off whenever you need them, separate from your main headlights. That's the whole point of fog lights.