Will this plug straight into my factory wiring harness?

These mount at factory points and use OEM electrical connectors. You should have a direct plug-and-play installation—no wiring modifications needed.

Does the heater work independent of the power adjustment?

Both functions operate through the same electrical connector, so yes—you can use the heat function separately from the mirror adjust controls.

What about the left mirror—do I need to buy both?

This is the right side only. If you need to replace the driver's side, order the left mirror separately.