Hi, guys this is my first post due to the fact that I have always found the answer by searching thru the threads. But no luck this time even with the vast amount of knowledge you all have shared. Anyways, the question is....
My blower motor comes on and off. I've checked the plug and the motor and know that they are both good. I don't think it's the switch due to the fact that when the motor is running I can adjust the speeds. When it's not working, the only way I can get it to come on is by moving the vent/air/heat/defrost selector back and forth from left to right. Any suggestions

 

You should start out by checking all the vacuum lines to look for crack lines.