This fall my 2002 Freightliner Columbia with a C12 has been operating with very low engine/water temperatures.  I've not once seen the gauge get above 175 and quite often its 125-150.  Outside temperatures last week were upwards of 90 degrees.   I thought I must have a bad gauge or sending unit.  This morning it was 45 degrees when I took my first load to the elevator and after 20 minutes of running the gauge was still sitting on 100 degrees and my heater only blew cold air.  I think my thermostat must be stuck open.  Is this a reasonable assessment?  Where is it located and any tips for changing it?