GM-GMC: Heater fan stuck on, 2003 gmc savana, blower motor resistor

Question
2003 GMC Savana.  When we turn the ignition on, heater fan comes on high even though set at low.  Can't turn it off.  Eventually stops by itself.  Also starting about week ago, started to overheat intermittently.  Turning on fan and heater to high brings temperature back to normal.  This happened last summer.  We had thermostat replace and that solved problem.

Answer
The blower motor speed levels are controlled by a blower motor resistor.  It is not uncommon for one of two things to happen, the blower motor resistor relay to stick, resulting in high speed at all times, or the connector that the resistor plugs into melts causing it to short out.  That can cause a number of different concerns including the symptoms you described.  
The resistor is located under the hood on Savana's on the passenger side screwed to the blower motor housing.
Hope this helps.