Haven't seen that one before, John! If you remove the screws in the back panel, there's about 3" of space to the back wall of the smoker. It's filled with insulation, and the back wall is sealed. You'll also see space above the inside box top, with insulation also. My guess would be that you either a) have a breach in the back wall (unlikely), or b) the fitting in the vent hole is loose. The vent fitting is threaded, and has a nut on the inside of the top. If this is loose, it could allow smoke/moisture into the top cavity, which would explain the drippings from the very top on the back.
Check that, and let us know. Might want to go ahead and remove the back, too, just to see the stain pattern.