Let's make sure the heater hoses entering the firewall are hot when your rig is at operating temperature, if not, it's possible the heater core is stopped up (no circulation) taking the hoses loose and pushing water from your garden hose in both directions thru the core may unstop it ('course, you said you had checked it already)
i'm banking on your hoses being hot, and the above procedure is not necessary! There are blend doors within the hvac unit that may be "stuck/ broken/ or the actuator just ain't-a-workin' to move them"
With a large (ain't I pretty!) Hand mirror and a good flashlight, you may be able to see the actuator (and whether it moves or not) when you move your controls.
I hope this aids you
let us know how things are working out!
The medic
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Sunday, January 18th, 2015 AT 7:51 AM