The best way to locate a heater core hose is to determine where the heater core is located. Many vehicles have the heater core inside the dash. Other vehicles, usually larger vans and trucks, will have a heater core box inside the engine compartment. Once you have determined where the heater core is located in your vehicle, that will allow you to easily locate a heater core hose. If the heater core is inside the dash, you'll need to examine the firewall for the hoses. If it is in a heater box, you'll need to look around the engine compartment for the box.
Heater cores located inside dashes can be very difficult and time consuming to replace. Depending on the layout of your engine compartment, it can also be difficult to locate a heater core hose. If there is very little clearance between the engine and the firewall, you may have difficulty finding the hoses. You might need to use a flashlight to look from the underside in some cases. You will typically be able to identify the heater hoses where they enter the firewall by looking for two two small diameter hoses that protrude into the engine compartment. You can then trace those hoses to their other ends, where they will typically connect to a thermostat housing, the radiator, or a t-connector.
If you can't see the firewall to look for the hoses, you can sometimes trace them back the other way. If a small diameter hose, typically between 1/2 and 3/4 inch (1.3 to 2 centimeters), is connected to your radiator, this will often be a heater hose. Similarly, if you can find a small diameter hose that is connected to your thermostat housing, you may find that this is a heater core hose. You may also locate a heater core hose by finding a hose that connects to the engine itself, or to a t-connection that connects to the engine.
It is often easier to locate a heater core hose if the vehicle has a heater box. This will usually be a conspicuous box located in one corner of the engine compartment that has two small diameter hoses extending from it. These will generally be the heater hoses.
In some cases you may be able to locate a heater core hose by looking for the heater valve. These applications use an inline valve to control the flow of coolant to the heater core and are operated by a mechanical cable that comes out of the firewall. If you can find this cable and trace it to the valve, you will have located one of the heater core hoses.