Does this install at the factory brake light location, or do I need to drill?

It mounts at factory points — no drilling required. Pop out the old unit, plug in the new one.

Will this work with my vehicle's existing wiring harness?

Installation method and connector type are not specified in the product data. Test-fit the unit against your vehicle's harness before finalizing; you may need an adapter or small rewire.

What's the return window if it doesn't fit or work?

Refer to modsupply.com's return policy page for your specific order. Most aftermarket lights carry a standard return period once condition allows.