The two tubes are connected to a duct under the rear dashboard that is part of the "Flow-Thru" air system (Not A/C) that a lot of the slab C-bodies came with. The lower end of the tubes go through to the underside of the car as drainage, as the louvers behind the rear glass are open to the weather.
I guess this system was pretty common for non-AC cars like mine? I suppose if you have air conditioning, it is not there.
Overall, it has always seemed pretty useless, as it's not a defogger, and it really doesn't move that much air; but at least it's operational i guess...