Actually in the W123, he is correct. The tube goes from the center of the dash via the foam tube, back to a hard rubber tube and to the right side of the evaporator box where it attaches to a port on the box so the AC/heater fan can suck air through the tube.

Dave

And this tube extends all the way to the fan housing?

Either way, It's pulling hot air off the dash to measure. On a hot sunny Texas summer day, the dash feels much warmer than the ambient air temperature. Looks like an easy spot to relocate to would be just left of the glove box below the wood trim. I could still drop the passenger side cover without disconnecting anything.

http://www.peachparts.com/Wikka/W123Evaporator

Picture number 64 shows the tube under my dash. 1982 300TD

Dave