Unless I missed something in this hijacked thread, there is no discussion with regard to the Auxiliary heater's switch. There are two versions, and I don't know which one Bulbasor has: (1) the older version of a standard Mercedes toggle with 4 or so vertical heat lines; and (2) the rectangle timer unit located next to the power plug. The Auxilary Heater (at least with the 2002's) requires the key in the #2 position, before toggling on the Auxilary Heater, turn the fan on, and the thermostat to something other than cool. The key can then be removed and the Auxilary heater will continue operations of 2 hours. The models with timers don't need this keyed action to get them started. I occassionally hear the heater turn on by itself if the heater switch is also on (middle bar) and the van is cold but running.
At least with the older models, REST has nothing to do with the Auxilary Heater. It is only a heat scavenger on a warm but shut-down engine.