I have a 2000 Cherokee. Up until 2 weeks ago the heater/AC fan worked fine, now sometimes it works and sometimes not. It has been very cold here in Boston for about 4 week straight, teens and colder at night. I turn on the fan and sometimes nothing, I leave the control on high speed and it has come on after about 15 - 20 minutes. One time my daughter slammed the door and the fan fired up.

Thoughts?
Thank you

 

Take a look at the plug down at the motor, I bet it's overheated and making poor connection.