Do I need to buy bulbs separately?

Yes. These lights come empty. You'll need to source 899 bulbs separately — OEM replacements or any brand-name 899 will fit. Some buyers prefer LEDs in this socket for brightness and longevity.

Will these work with the factory wiring harness?

These mount at factory points, but confirm your harness connector type before ordering. If your Mustang's original fog light wiring is intact, these should plug in. If wiring was removed, you'll need to run new harness.

Do I need to remove the bumper to install these?

Depends on your comfort level. Most installations allow access from underneath or behind the bumper opening. Full bumper removal makes it easier but isn't always necessary — test-fit first to see what works for your setup.