I did the factory heated mirror install on my SR5, and yes we have the pre-wired connection. However, you need to splice into the two unused wires to power the heater elements. There are two connectors on the back of the mirror glass for the heating element. Run two wires from the mirror connectors through the mirror housing like the other power mirror wires. Find the two unused wires on the connector in the door. Cut the wires about 1.5" back from it and splice in the new wires from the mirror. Before you wrap everything up, turn on the rear defrost and feel the mirror glass to see if its warm.
Before anyone bothers with the "This Post Is Worthless Without Pics" post, I don't have pics because my camera is out of service. If you need instructions on the removal of anything, check on Burtman Industries website for the Muth heated signal mirrors installation instructions. It will get you to the point where you tap the power mirror connector in the door. PM me if you have other questions. Good luck.