Do I need the mirror housing or backing plate to install this?

You'll be swapping the glass into your existing mirror housing. Make sure your current housing is in good shape before ordering — this is glass only.

Will fasteners come with this, or do I need to source my own?

Hardware is not included with the glass lens. Check your existing mirror housing to see what fasteners are already there; you'll likely reuse them when you swap in the new glass.

How do I know if I'm ordering the correct side?

This is the right-side lens. If you're standing in front of the vehicle looking at it, right side is the passenger's side mirror. Confirm your mirror location before checkout to avoid ordering the wrong side.