Will this lens fit my existing mirror housing, or do I need a whole new mirror assembly?

This is a glass lens replacement only — it installs into your current mirror housing at the factory mounting points. No new assembly needed.

What if the glass is cracked but I'm not sure if it's the lens or the backing?

Pop your mirror cover off and inspect the back. If the mirror backing (reflective silver coating) is damaged or peeling, a new glass lens alone won't restore it — you'd need the full mirror unit. If the glass itself is just cracked, this replacement will work.

Do I need any tools or adhesive to install this, or does it just bolt on?

Mounting method not specified — test-fit your housing before committing to fasteners or adhesive. Most factory mirror housings use a clip or adhesive backing; verify which your Park Avenue uses before installation.