Do these mount at the factory fog light holes, or do I need to modify the bumper?

These mount at factory mounting points — no drilling or bumper modification needed. Bolt-on means you're using existing holes and fastener locations.

What bulbs do I need to buy separately?

899 bulbs. Pick up a pair when you order — they're standard and cheap. Make sure you grab the right wattage for your setup.

Will these work with my existing wiring harness, or do I need to run new wire?

Wiring setup depends on whether your truck came with factory fog lights and what harness you have. If you're starting from scratch, you'll need to run new wire and likely a relay — that's not included here. Test-fit the assembly first to confirm your wiring plan.