Will this bolt straight onto my existing mirror housing?

Yes. This lens mounts at factory points on your OEM mirror housing — no brackets or adapters needed. Pull the old lens, align the new one, and secure.

Do I need to remove the entire mirror assembly from the door?

Not necessarily. You can swap the lens with the mirror still mounted to your door. Just support it so it doesn't fall while you're working.

What if the lens won't seat properly after I install it?

Stop and inspect the mounting tabs on both the lens and housing for debris or damage. Clean with a dry cloth. If tabs are bent, the housing itself may need replacement instead.