Hi Brian, thank you for reaching out. Typically, the black wire is negative (ground) and the other wire is positive. Since the new mirror’s wire orientation may differ, you can identify the correct polarity by checking the original mirror’s wiring or using a multimeter to measure voltage when the heating function is activated. If unsure, carefully test the wires by matching the original connections or reversing them until the heating element warms up. To avoid damage or electrical issues, it’s recommended to confirm polarity before final installation or seek professional assistance if needed. We thank you for your purchase and have a wonderful day!