Will these mount at the factory fog light locations without drilling or modifications?

Yes. These are designed for factory mounting points on your Yaris hatchback. No drilling or bracket fabrication needed.

Do I need to splice into existing wiring, or does the switch handle everything?

The integrated switch is included, but you'll need to run power from your battery or fuse box. Refer to your vehicle's wiring diagram to identify the correct harness points for cleanest integration.

Can I replace just one lens if it cracks, or do I have to buy the whole pair again?

Replacement policy not specified. Contact support with photos of the damage to confirm if individual lenses are available separately.