Air Conditioner Relay or Switch

2003 GMC Sierra 6.0 V-8. manual dual climate control. Part of the time the air works, part of the time the air does not work. If you start the truck and the air comes on it will not quit (even 8 hrs of driving it did not quit). If you start the truck and the air does not work, it will occasionally start working and occasionally will not work.
Does this sound like a switch problem or a electrical problem under the hood???
Thanks