Do these mount using factory holes, or do I need to drill?

These are designed for factory mounting points—no drilling. Drop them in where your OEM headlights were and bolt down using the existing holes.

Will I need a wiring harness or adapter for these LEDs?

Wiring compatibility isn't specified in the product data—verify with the seller or test-fit before finalizing installation to confirm connector type matches your Mustang's harness.

Can I paint or refinish the chrome housing if I want a different look later?

The chrome is a factory finish. If you want to modify it, you'll need to sand and prep the surface before applying new paint or coating—standard bodywork rules apply.