Heating Tech Problem!!!!

Well hello everyone

I have 2003 gmc yukon SLTw/Autoride and it has aux. heating in the back for passengers. Well my heating works from the dash but, not in the rear??? can anyone help!!

Thanks

Was it working before? Did it just all of a sudden stop working? I would check fuses under hood first to make sure that they are all good. Nice diagram of what is what on underside of fusebox cover.