If it has a heat control valve, that will be in the inlet hose. The arrow indicates the direction of flow. So the arrow would point away from the engine towards the heater core when installed in the inlet line.
As a general rule of thumb, a heater core with the same sized lines, you install the inlet hose on the fitting that feeds the bottom section of the core as that tends to push the air out of the core as it fills.
The return line fitting on the engine is 99% of the time the one closest to the water pump. That helps draw the heated coolant through the core providing better heat.
On a vehicle that has different sized heater hoses, the smaller line is the inlet side. That is done because the incoming coolant is under pressure. By increasing the size of the return line to the water pump it lowers the pressure in the core and makes it less prone to leakage, it also reduces the noise that the coolant creates as it flows through the core.
Another advantage to the different sizes is that during assembly, it is hard to mess up and install the lines the wrong way!
Jan 11, 2017 at 10:22 PM
