Had the same problem a few weeks ago with my 04' SVTF. Saw the leak from the o-ring at bottom of housing as your describing. Changed the thermostat housing, o-ring, and new thermostat, since I was already in there and it still leaked. Found the leak was coming from the sender at the top of the housing. Had to buy the whole sender assembly from Ford because they don't sell just the internal rubber o-ring that seals it. No more leaking...