Do these connect to my truck's existing wiring, or do I need to run new harnesses?

These are designed for plug-and-play integration into the factory fog light circuit. If your truck has factory fog locations pre-wired, you're golden. If not, you'll need to run power and ground from the battery or fuse block — standard auxiliary lighting install.

Will the switch mount in my existing dash cutout, or do I need to drill a hole?

The rocker switch is included, but dash fitment depends on whether your Silverado's interior already has a fog light switch opening. Test-fit before committing — if there's no factory opening, you'll need to drill or use a switch pod.

Are bulbs included, or do I need to buy them separately?

Bulbs are not included. You'll need standard fog light bulbs for your year — ask your parts supplier for the correct fitment and wattage spec.