Does this require the matching left-side DRL to function properly?

This is the right-side unit only. For a complete front daytime running light setup, you'll need to purchase the matching left-side assembly as well.

Will this work with factory wiring, or do I need a harness adapter?

Mounting method and wiring integration are designed for OEM fitment points. Test-fit before finalizing any electrical connections to confirm compatibility with your specific vehicle setup.

What's the return window if this doesn't fit or work as expected?

Standard modsupply.com return policy applies — check the policy page for exact timeframes and conditions. Keep your receipt and packaging in case you need to initiate a return.