Will this plug directly into my existing harness?

Yes, this uses the factory connector and mounts at the original brake light location. No wiring modifications needed.

Do I need to remove the tailgate to install it?

Mounting method not specified in the product listing. Check your service manual or test-fit the assembly before you're committed to any disassembly.

Can I use this if my OEM light is cracked?

This is a direct replacement for the factory third brake light. If your mounting points are intact, this will bolt in. Verify the bracket isn't bent before ordering.