Ya so the AC still does not work.....my Expy has rear air....and I see it has a leak on the rear drivers side AC unit...is that a condenser behind that panel? Whatever it is I see a leak it bubbles when the AC is on.
Any ideas?
...Gage

 

I have Lincoln Navigator 2006 and has the same hissing sound coming from back side. later the front AC also went off. I took it to mechanic and he has to open the right rear side décor and the rear seat belts along with the rear side décor to open and see what leaking
be careful in opening the décor as there may be some bolts are hidden.
once its open , you can easily see the leak. in my case it was the valve where oil was visible . thus I had to get valve and washers. the OEM valve comes for just 32$. thus always go for it.
after placing valve and washers, keep the pressure in ac tubes for one night to check for any other leak. if its okay , recharge the Freon and put all décor back