Will this plug straight into my vehicle's electrical harness?

This is a factory assembly, so it connects via the OEM mirror harness under the door panel. Your existing wiring should mate directly—verify connector type before purchase to ensure compatibility with your specific vehicle.

Can I replace just this mirror, or do I need to buy both sides?

You're buying the driver-side mirror only. If your passenger mirror is also damaged or outdated, you'd source that separately. Many buyers purchase one at a time as needed.

Does the heating and signal function work when I receive it?

This assembly ships fully functional. Power and heat test it before installation—if either feature doesn't operate, flag it immediately for return.