I recently bought a '97 S420 with 24k miles from the Houston area. It appears that none of the floor vents are open and the center console "flow through vent" only produces cold air even with the Auto / EC activated in ambient cold temperatures. I wonder if the heat has ever been used in the car? Any help or suggestions would be most appreciated!

 

I also recently purchased a 140 series (94) S500L. Also low km - here in Sydney (Australia). Similar climate concern. Same problem with heater. It ended up being the main heater valve/ distributor. It was frozen shut and needed to be replaced. Local cost - around A$1,000. Now works perfectly. Good luck.