So - the evaporator must be to blame. Since there are quite some chemicals you can use to clean the evaporator, the only problem was to find an entrance into the housing (plenum) of the evaporator (and heater as well - they are in the same housing) to use such a spray.
BTW: please note, that the best way for a proper cleaning in order to tackle the smell problem is to get as close as possible to the evaporator. Although there are chemicals as an aerosol you can spray into the air inlets (not that of the motor of course !) or into the recirculation inlets in the cabin, they all cannot beat a thorough cleaning job at the center of the problem: inside the housing of the evaporator.
You will find the evaporator housing behind and under the dash at the passenger side. In order to get a proper access, just remove the glove compartment by first pressing the left and right side of the compartment inwards, so it can be moved along the dash forward. Then remove the three screws at the bottom side of the glove compartment, so you can take it out.
When searching for access to the housing, there are referrals to the blower motor resistor or the temperature meter that goes into the housing. However, these are very difficult to remove and not easy accessible. Therefore the most effective and efficient solution is to drill a hole into the housing so the spray tube fits through (the tube with nozzle is normally sold together with the spray can).
That´s what I did - see picture below (hole is left above of left above tape corner).
Make sure that you drill a small hole at this area (or somewhat higher eventually), but not to the left or right or you might hit the heater or evaporator in the housing (see http://pics.bbzzdd.com/users/sluggo/dash3.jpg showing picture where they are located: heater = left, evaporator = right).
So, now you have access to the housing, follow this procedure to clean the evaporator:
1. Remove Fuse F30 (a/c clutch relay), which is located in the Battery Junction Box under the front hood. This will make sure, that the compressor will not start during step 2.
2. Put the system on RECIRC (recirculation), maximum temperature, maximum fan speed for 10 minutes. This will dry the evaporator, so the chemicals put in in the next step will do their job as good as possible.
3. Bring the tube with nozzle inside the housing (there is about 1 inch free space behind the housing wall when you make the hole where indicated) and move the tube around while bringing in the chemical. Make sure you have some towels on the floor underneath the housing (in case of spillage) and also make sure you have something on the floor underneath your car (in case of spillage through drain eventually).
4. Close the hole by duct tape when finished and replace the fuse F30.
5. Follow the instructions on the spray can for follow-up procedures (like running the system with open windows for 15 minutes or alike, depending on the type of chemical product).
6. Put back the glove compartment (in opposite order, see above).
Please be aware, that in the 2007 Ford Expedition there is no cabin air filter ! In case you have a car with such cabin air filter, remove it before applying the chemicals, and put back when the cleaning job has finished (or replace the filter as weell, which generally speaking should be a good idea anyway).
This procedure worked for me very well. And since there is an easy accessible hole, I can repeat the cleaning when necessary without any time consuming operations.
Hope this information to be helpfull. Since I live in the Netherlands, there are not much experts for a 2007 Ford Expedition EL Eddie Bauer overhere, so I am very happy with your forum !