BOPE ;
Can’t find the link to a PP post on the rear defrost as I recall someone posted their hand drawing of the wiring terminal connections.
I incorrectly mentioned 3 stage but its actually 2 stage, the dash knob pulls out 2 notches to operate either center heat grid or all 3 (top and bottom) grids.
Sometimes those metallic brown lines in grid are damaged from age exposure and no longer work to heat the element.
Maybe this link will help it's a bit similar if you haven’t already read it :
http://forums.pelicanparts.com/porsche-911-technical-forum/143220-where-do-rear-window-defrost-wires-unplug-2.html