Will these bolt to the factory fog light mounting points?

Yes. These mount at the stock locations with no drilling or panel modification needed. You may need to remove the existing trim ring or blank panel depending on your current setup.

Does the switch wire into the existing harness, or do I need to run new lines?

The switch assembly ships with harness leads ready to integrate into your vehicle's electrical system. Route the wiring along the existing loom behind the dashboard. If your Sienna has no pre-wired fog switch location, you'll need to tap into fused power and ground directly.

Should I test the bulbs before final installation?

Absolutely. Power up the switch and confirm both lights illuminate before you button up the bezel and trim. If one side doesn't light, swap the bulb first—defects are rare but easier to catch now than after reassembly.