Why won't the blower motor run on my 1999 buick century until the engine gets warm?

I recently bought a 1999 buick century and have had problems with the heater not working properly.

If the fan knob is on high when the engine is started, the fan will come on. If I try to turn the fan on while the engine is running it won't come on. No heat nothing. The guy I bought the car from said that the 1999 buick century had an option that kept the fan from blowing until the engine was warmed up. Is this true?Why won't the blower motor run on my 1999 buick century until the engine gets warm?Some Buicks have a optional temp contol which regulates the temp in the car automaticaly.The sensour may be dirty or the air flow to it maybe blocked.Its normaly on the dash a small vent hole.Why won't the blower motor run on my 1999 buick century until the engine gets warm?I have an Olds Ciera Cutlass which is very similar. The fan works 90% of the time. I am still clueless about the other 10% of the time.