Does this mirror connect to the existing wiring harness?

Yes, it's a direct OEM replacement and plugs into your vehicle's factory wiring. No rewiring needed if your Altima was originally equipped with powered mirrors.

What if the power motor doesn't function after installation?

Test the mirror adjustment switch before finalizing mounting. If the motor is unresponsive, verify power is reaching the connector and check your vehicle's fuse box for a blown mirror fuse.

Will this fit if my Altima has a manual mirror on the other side?

This is a powered mirror assembly only. If your passenger side was originally manual, you'll need to verify your vehicle's wiring supports powered operation before installation.