In our experience with the '98 ML320 we owned we had the same problem for the driver side footwell. We used the 6 o'clock heater control position, used fan position one (so the air movement didn't feel as cold), closed all the dash vents whenever we didn't need them and plugged the passenger side footwell outlet next to the center console with a shop rag. The passenger side well was still getting plenty of heat. We did have the repair modification mentioned by Dr. Diesel that helped some but did not solve the problem entirely so we continued to use the shop rag modification.

Jim W.