Are there fuse holders (metal tabs) at the bottom of the fuse box that would physically hold a fuse in the slots you speak of? If not, the vehicle is not wired for it. Positions 8 & 9 in the underhood box. Probe with a test light or voltage meter with the ignition in the ON/RUN position if you can't see inside. Somewhere there is also a relay, as the glass warms for like 6-8 minutes and automatically goes off.
If you were to find the glass new, Ford's part number was DW-01619-GTY and the cost is somewhere $600-$700 retail. Its been long discontinued but they are still out there. There were no aftermarket suppliers, so nothing from Safelite, PPG, etc. Carlite was the OEM producer. Wasn't a terribly popular option to begin with, so probably super difficult to find one in a salvage that isn't broken from being in a wreck. Not that salvage yards are exactly loaded with 4th gens to begin with.
Its a very cool feature to have, but does have its drawbacks. In the early morning sun or late evening sun, the heating element grid can reflect and its pretty annoying. Driving at night, the grid lines are very apparent under the street lights. I'll dispel some myths I've read about heated windshields -- my radar detector works just fine, as does my turnpike electronic payment device.
Not worth pursuing IMO.