Will this harness work with factory headlight wiring, or do I need to tap into the fuse box?

This harness connects directly to your factory headlight connector — no fuse box tapping required. Plug the harness into the OEM socket, then run your aftermarket bulb leads through it.

Do I need a relay or capacitor module for the DRL function to work?

The harness itself is passive wiring. If your Boomerang housings require external DRL control logic, check with the light manufacturer — some run on factory headlight power alone, others need auxiliary modules.

Can I use this with STI headlights, or is it WRX non-STI only?

This harness is designed for non-STI WRX models with non-LED factory headlights. STI housings use different connector pinouts — confirm your specific headlight style before ordering.