Ok: I'm not so familiar with how GM does it but the fan in auto mode will try not to freeze you or make you too hot in systems with ATC. Usually the defrost setting just lets it go but this one may not. In many they are sensing the available temp in the inlet heater hose and wait till it gets up to perhaps 120 degrees F before if will engage fan but will run anyway in defrost.
If you find the inlet hose and a block with just two wires that would likely be the sensor. AYOR, that if jumped might defeat the delay idea but the rest should work. Again - that's At Your Own Risk - it works in older Fords but can't say for GM.
If you can see the body of the blower motor perhaps just giving it a good tap will get it to go and that would indicate the motor itself is a bit sticky and should then be replaced. That's common but not always so consistant as you indicated. Motors that respond the the tap trick have problems with the brushes making contact,
T
Tom Greenleaf - MetroWest Boston - USA
http://www.acsource.com/ _________________________________________
http://www.autoacsystems.com/tomgreenleaf/
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