Do I need to run new wiring from the battery?

The switch mounts in the housing, so you'll need to run power to the assembly and ground it. Refer to your vehicle's service manual for the correct circuit and gauge wire.

Will these fit my Civic if it didn't come with factory fogs?

These mount at factory fog light locations. If your Civic doesn't have the mounting bracket or wiring harness from the factory, you'll need to source those separately or fabricate mounting points.

Can I replace just one bulb, or do they come as a pair?

The assemblies ship as a matched pair with both bulbs included. Bulbs are user-replaceable once installed—source individual H8 replacements as needed down the road.