When I turn on heater, with face vents on, somewhat warm air comes out I change it to face/floor, it only comes out the dash, and makes a funky thump sound when it's switching over, and doesn't come out at the floor Same with floor/defrost & defrost, we pretty much keep it on floor/defrost bc it seems to blow warmer than it does out of the face vents.
As for the rear, sometimes It comes on for me, sometimes it doesn't. But when it does, it's warm.

 

Check the electric actuators under the dash; these do go bad. Typically there are 3 of these to deflect the air flow.

I had one go bad on our 1994 Suburban, and same on our 2005 Yukon.