Just in the past couple weeks I have been experiencing an issue with getting air out of the vents. For the front they will work more consistently but it does not always blow at the selected speed. The rears will turn on randomly then shut off again. I do notice that for the first start of the day front and rear will blow as expected but go out thereafter. I've been trying to research similar issues but usually it is only regarding the rear. Any ideas on where to look first? Being that this started at the same time, I'm guessing whatever failed is a shared component. This is on a 2001 Yukon SLE.