I got home from work on Friday to find that the Durango was leaking
antifreeze on the driveway.  I discovered where it was leaking, but had a
real problem finding out what the part was called.  

After much searching online - since it didn't show up in AllData, I found
both the name and part number.  It is called an "Auxiliary Coolant /
Water Pump". 
It is located off the heater hose between the main engine water pump and
the "Y" between the front and rear heater cores.  The truck has
two separate heater cores - one for the front and one for the rear
controls.  

The pump provides the extra pressure to push the water to the rear heater core.  
For the time being, I bypassed it with a 5/8" coupler and a few screw
down clamps.  The part arrived today, so I'll be replacing it this weekend.

- Mark
  
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