Flow through the core is in from the bottom, up through the coils and out the top.
The bottom hose is the higher pressure hose and connects to the nipple on the intake manifold.
The return to the engine is the outlet from the top of the core and can connect to either the tapped connection on the water pump or the nipple on the top of the RH side of the radiator. Schematically speaking, in the system the are in the same run of pipe.
Inlet hose is the smaller (9/16"?) of the two - red in the image below.
Outlet is 3/4" and is shown as black below.
Wait a second:
By the truck will not blow hot air!
When you say this, do you mean air blows from the vents but it just doesn't get hot? Or, do you mean that there is no air flowing at all?
If there is air flow, but it's just not getting hot - continue reading.
If, on the other hand, there is no air issuing from any of the heat, A/C, or defrost vents, the problem is electrical - not a heater hose/core or flow issue. Come back and provide more details.
Check that your engine is up to temp, and that there is coolant flow through the heater core. The dash gauge will suffice for engine temp.
To check for flow through the core:
With the engine at normal operating temperature and running, set your heater controls to max warm. Have the blower on high. Leave your doors open.
Open the hood, reach in, and grasp the heater hoses - one in each hand. It might take a few seconds but they quickly should feel hot to bare skin.
The actual temperature doesn't matter at this point. The main thing is that you should be able to feel a distinct difference in the temperature of the 2 hoses.
And, it needs to be like this: the inlet (bottom hose) should be considerably warmer than the outlet (top hose).
The difference in hose temp will vary depending on the ambient air temperature. But, even on a warm day, you should be able to detect a loss of heat in the coolant as it passes through the core.
