Will this fit my stock bed, or do I need a tonneau cover?

This mounts to your factory third brake light location — tonneau covers don't affect fitment. If you're running a bed cover, confirm it doesn't obstruct the rear window area where this sits.

Does this come with a wiring harness or do I need to splice into my factory brake wires?

Wiring configuration not specified — you'll want to test-fit first and confirm whether it uses a plug-and-play connector or requires hardwiring to your factory brake circuit. Have a helper verify brake light function before finalizing.

Can I return this if it doesn't work with my setup?

Check the return policy at checkout. Since this is a brand new OEM-replacement unit, inspect fitment immediately after arrival — don't delay if there's a mismatch.