Good news. I gently removed the rear slider and vent grill (the whole thing just pops out). The derailleur cable somehow worked its way out of the holding clamp. I separated the cable under the console (it splits in two halves) and yanked out enough slack at the rear, then re-clamped and reconnected the two halves. Buttoned everything up, and now have COLD air conditioning blowing in the back! It seems to be tied in to the front center vents too, so if you close them about half way you get a strong stream of air from the rear of the console!

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280