I'm having a strange problem with my 1987 260E's heat. On cold days (below 25F) the heat will not turn on unless the car is warmed up and then shut off. Once you start it back up the heat works fine. When the heat is not working, the blower motor is not spinning and no switch does anything on the control unit. Once you restart the car though, the heat works on all settings. Also, the heat works fine on warmer days above freezing.

Any ideas? I'm thinking either the control unit, or the temp sensor in the head?