Do these bolt straight into my factory headlight bucket?

Yes — these mount at factory upper and rocker points with no drilling or frame cutting required. Your original harness will plug in directly if your Mustang still has the stock wiring.

Will I need new wiring or a relay harness?

LED draw is minimal compared to halogen, so most stock Mustang electrical systems handle it without modification. If your alternator is undersized or you're running heavy auxiliary load, a relay kit is smart insurance — but not mandatory for these lights alone.

What's the return window if they don't fit right?

Test-fit before final installation. If there's a fitment issue, contact ModSupply customer service with photos of the gap or misalignment and we'll work out the next step.