So after upgrading my Tacoma, I couldn't resist doing it to the family scooby as well.
As always, proceed at your own risk, I in no way assume responsibility for mistakes in this article.. If you don't feel comfortable performing this work: Don't! Or ask some questions clear it up. Proceed at your own risk!!
Of course the 09 forester was a little bit more challenging, as in not as much spaced out as the 02 Taco, and a little easier, as in I only had to tap in to the Rear defrost circuit to make it happen.. Here goes.
I installed Burco #3801 heating elements into the mirrors. In short: The mirrors have to be removed from the car, to route the wires thru. The mirror & plastic backing have to be removed to install the heating element. The doors panels have to be removed in order to do this.
Our 09 forester 2.5X has two spare wires that run from the connector inside the cab to the connector near the mirror (the black w/white stripe, and Red wires). It appears that Subaru uses the same wiring harness for the doors, and for the main body harness omits the mirror wires.
I have the 09 FSM wiring diagram, and almost thought that all I'd have to do is install the heating elements, and wire them to the door harness, however, found that the wires in the door connector didn't have power when the Rear Defrost switch was activated.
Disassembling the doors:
1: pop off tab on door opening handle tab, remove screw. Same on handle.
2: Two more screws are hidden under the power window switches. This silver trim panel pops up, I used a flat head screw driver, slid it under the back end, and popped it off, it takes more force then you think! I thought I'd break it, but that didn't happen.
3: After all 4 screws are out, you can pop the door panel off, it has perhaps 7 of those fasteners, start at the bottom and work your way around popping them out of their holes, then lifting the panel up and out.
4: you can leave the door panel hanging by the cables for the latch, didn't seem to hurt ours. You'll have to unplug the wires for the 'paddle light', and the door lock and window switches.