I hope you are referring to the vents UNDER the front seats that point to the rear footwells:
They serve HOT air to the rear passenger's feet.
In W124s with the full automatic A/C system, like all U.S. cars come equipped with, you will only get air down there if the heater is working (turn the temp rocker switch all the way to the red area).
In Euro W124 with the regular A/C where you can can control the temperature independent from left to right, you also have more control on WHERE to direct the air, so you may apply cool air to the rear footwells.
With my W124 E320T I made the stupid mistake of overpaying for the full automatic A/C systems. Now I really regret it, since the regular A/C system on my previous W124 300TE worked excellent and gave you more control on what type of air and where to send it.