Will this work with my existing wiring, or do I need to modify anything?

This uses the factory connector, so it plugs straight into your OEM harness with zero modification. If your car's brake light circuit is functional, you're good to go.

Do I need any fasteners or adhesive to mount this?

It mounts at the factory brake light location on the hatchback. Bring basic hand tools and verify the fastener condition in your car before install — you may reuse existing hardware or pick up new fasteners locally if needed.

Does the LED assembly come as one piece or do I need to install bulbs?

The LED array is integrated into the assembly — no separate bulbs to swap. Install the entire light unit and you're done.