Not trying to be a pedantic, pontificating, pretentious....
Actually, this is the evaporator drain tube. As the moisture laden air passes through the a/c evaporator core, the moisture condenses, and needs a place to run out. Without this tube, the water build-up in the heater/evap case would eventually run out on the floor board. These drain tubes have been in use for decades. Boggles my neurons when someone brings this up (IMG:
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BTW, the condenser is in front of the radiator and needs no drain tube..