Do I need to remove the entire mirror assembly to install this?

No. This lens is designed to swap into your existing mirror housing from the front. Depending on your mirror's design, you may need to remove trim pieces or the mirror backing, but the mirror bracket stays mounted to the door.

Will the heater work right away or does it need any special wiring?

The heating element connects to the factory mirror heating circuit in your door harness. If your vehicle already has heated mirrors, it'll activate through the OEM switch. If not, you'll need to verify your door wiring supports the circuit before installation.

What if the lens doesn't sit flush after I install it?

Test-fit the lens before fully seating it. Check that no trim clips are bent or blocking the housing. If the glass wobbles after installation, remove it and verify the housing backing plate isn't damaged. Contact support with photos if the fit remains loose after checking hardware.