I found this tech article and discovered that the foam hose connected to my ACC temp sensor behind the dash is totally shot:
http://www.mbca.org/pages/tech/MBCA_acc_system.htm
The article says to replace the stock foam hose with foam pipe insulation from the hardware store. But do I have to use foam at all? Why foam? Why can't I just use a piece of rubber heater hose, for example?