Will this fit into my existing brake light housing?

Yes, this assembly mounts directly into the factory location. No drilling or housing modification needed — pull the old one and bolt this in.

Do I need a wiring harness adapter?

Check your truck's existing connector before ordering. Most factory harnesses mate directly, but some years may require a simple pigtail harness to avoid splicing into the original wiring.

What's the brightness difference between this LED and the stock bulb?

LEDs are noticeably brighter and respond instantly with no warm-up time. You'll see the difference in daylight and poor weather, especially when braking in traffic.