dopeless

I have an 04 RX 330 that I just bought used. I am having a problem. The temperature selector will blow hot air unless it is set to the "Lo" cool position. Any other setting on the temperature selector will only produce hot air from the vents. This occurs when the system is set on the "Auto" position. It has dual controls, and if using one side on "low" and the other side at a "temperature" (65-85) it will work but, the temp setting will blow fully hot. I have researched and probably have a blend door issue, but I don't here the sound coming from under the dash. Any suggestions?