There's no prewiring involved. Just as long that there is a power wire for the power mirrors. I've seen both set ups and they are exactly the same. If you don't have power mirrors to begin with, then that would be a lot of work to make them work.
you can splice off the pre existing power supply as long as you are sure it is constant, but you will have to use a switch to energise the heater mechanisms or they would stay hot all the time and that could lead to problems
I would wire it all from scratch so that you are working with an appropriately fused power source and you, pick the placement of the switch. Or better yet" brainstorm" splice it off your rear window defroster switch and up the fuse size. You will probably want the mirrors and the rear window clear at the same time. Let me know if that helps any!
I've got the heated mirror/powered mirror/puddle lamp combo too.
But if I was to try to add heated mirrors, I'd first open up the mirrors and check out the wiring harness. There might be already be wiring and connectors.
I'd also get a Haynes or Chiltion's manual and check the colors of the various wires. This might tell you if there is already wiring to the rear defroster in place.
Years ago, you didn't get anything you didn't order with the car. But the Japanese started putting all the miscellaneous hardware and wiring the factory or dealer *might* need, in all cars. Maybe Ford got a clue.
But if you find that out, you may save yourself a of time, not to mention blown fuses.
I beleave that it would be just plug and play. The easiest way it to unplug the mirror wire connector. Look for a DG/VT wire in the third pin on the top bay. If you have the connector upside down you would accedently be on pin 6 which is the left/right movenent, not the right one. See if it is switched on and off with the rear defroster switch. This is were a voltmeter comes in handy.