Will this plug directly into my existing mirror harness?

Yes. This uses factory mounting points and harness compatibility with Super Duty platforms. If your truck came with powered mirrors, the connector should match—no rewiring necessary.

Does the heater work on its own, or does it need the engine running?

The heater operates through your truck's electrical system and draws power when the ignition is on or the engine is running. Check your owner's manual for any climate control or defrost system integration specific to your model year.

What if the turn signal doesn't light up after installation?

Verify the signal bulb is seated properly inside the housing and that your truck's bulb type matches the mirror's socket. If bulb condition is confirmed, test continuity at the connector to rule out wiring issues on the truck side.