Do these bolt straight into factory fog light holes, or do I need to modify the bumper?

These mount at factory fog light points with no drilling or bumper modification required. Test-fit before final fastener selection to confirm alignment.

Is the switch plug-and-play, or do I need to solder anything?

The switch comes with a wiring harness. Run it to your battery and ground per the harness instructions—no soldering needed if you use the connectors provided.

Can I run these fog lights with the stock electrical system, or do I need a relay?

The harness is engineered for your Yukon's electrical setup. Follow the wiring diagram included with the kit; a relay is typically recommended for LED draws but is not always required depending on your truck's existing circuits.