All the air comes from the air handler in the back on the right side (looking forward) passenger side in the U.S.. There is two cores back there if you have dual a/c and heat. There is also a servo motor that rotates a flap between the evaporator on the a/c and the heater core. There is also another one that controls the air to top vents and the bottom vent by the right rear door. The side panel will come off to look at all this pretty easy. The first time it might take a little while. Hope this helps.
1995 Suburban K2500 6.5TD 3.73's Innercooled, Updated cooling, Banks 3" exhaust no gizzards in the kittie. Pyro/Boost/Trans temp. Transgo HD valvebody kit.
2002 2500HD, CC, LB, 4X4,California Truck, Federal Emissions Front pipe, Plugged recirculator, modified air box(Per Kennedy's page) 6" RCD lift, 35" tires.