Will these splice directly into my existing wiring, or do I need a relay harness?

The switch is pre-wired and designed to integrate with your factory electrical. You'll tap into the harness at the firewall or under the dash — exact routing depends on your comfort level, but no external relay is required for basic operation.

Do I need to drill the bumper, or do these use the factory holes?

These mount at factory cutout points — no drilling or modification to the bumper required. If your bumper doesn't have pre-cut openings, you'll need to carefully cut them to size before installing.

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

Bulbs are not included — you'll need to source the correct replacement bulbs for your Odyssey's fog light specification before installation.