I agree that using the rear heater for 5-10 minutes does a good job of
warming passengers sitting on the rear bench (for 20-45 minutes).  It
works even better when there is a blanket over the lap and legs of the
rear passenger(s).  The blanket really holds the heat close to the
passenger(s) and warms legs and lower bodies very quickly.  Much of the
heat stays trapped long after the heater and engine are turned off.
Lee Kouns
Ski vagabond