Will this fit if I have the factory spoiler on my hatchback?

Yes — this assembly is designed specifically for hatchbacks running OEM spoilers. No modification to the spoiler or trunk seal needed.

Do I need to splice into my existing wiring?

No. The harness plugs directly into the factory brake light connector. If you're replacing an existing unit, disconnect the old one and plug this in.

What's the difference in brightness compared to the stock filament bulb?

LED arrays output significantly more light at the same power draw and turn on instantly. You'll get better visibility from following traffic, especially in daylight or low-light conditions.