Does the heater hose have any valves that need to open and close?

Is the system full of coolant ? Is the heater control valve opening an allowing coolant to go through the heater core ? Under the hood in engine compartment , on the heater hose , a metal or plastic round canister with vacuum hose hooked to it .You can google heater control valve for 2005 ford explorer to see what it looks like . When you find it ,with engine running pull the vacuum hose off to see it the valve opens an closes ! Feel the hose after the valve , is it hot ?

don't know this van specifically, but some things to check, make sure cabin filter is replaced or clean, you didn't say if the blower itself works, if not, correct or replace. Check the hoses going to the heater core, if hot water is flowing thru the core, both hoses should be hot. Check the heater controls to make sure doors in the system are opening closing as they should, you may just be drawing in cold air and not going thru the heater core.

OK.
Start by warming up the engine, and then shut it off when it is up to operating temperature, and open the hood.
Carefully feel the heater hoses to see if they are hot.
If you are unsure of which they are, ask someone who knows.
They are about 1 inch in diameter and are found at the rear of the engine attached to the firewall.
If they are hot, the problem is likely the flap inside the heater inside the car, which is controlled by a vacuum line or a wire cable.
Someone will need to dismantle the heater box under the dashboard to see about the air flap.
If they are not hot, feel the radiator hoses to see if they are hot.
If the radiator hoses are hot, and the heater hoses are not, you will next check the hot water shut off valve which is in front of the firewall, and in between 2 pieces of one of the heater hoses.
This valve is controlled by a lever on your dash board, that is marked hot or cold, and may have the colors red and blue on it.
Those valves usually fail by "sticking" closed, because lots of people only use the heater a few months of a year.
The valve is opened and closed by a wire cable or a vacuum hose.
If the valve works freely, you may have a "stuck" thermostat in the engine, which would not make enough heat to warm you.
Nowadays, engine thermostats usually stick open when they fail.
A symptom would be the radiator hoses and the radiator are warm but not hot.

Lots of things could be "the problem". First, is the fan turning? Is the heater core on the passenger side under the dashboard getting hot? Are the heater hoses connecting to it BOTH getting hot? Is the on/off valve switch opening and closing as you slide it on the dashboard? If any one of these things are not happening you need to find out why!

You can purchase a manual valve at your local auto parts store, that you can install with 2 hose clamps on your feed line to the heater core. When you get the valve, just cut the line, place on the hose clamps (one on each side of the splice) and install the valve, by sliding the hose onto the valve. tighten the clamps tight, and close the valve. This will cut the feed to the heater core. Takes about 10 minutes to do. If you need the heater, remember to open the valve. thats it. Hope this helps

Thermostat stuck open, heater core clogged, or heater control coolant flow valve stuck closed. With the engine at full operating temperature check the hoses at the firewall going to the heater. If one is hot and the other is cold or just barely warm, the heater core is clogged or the coolant flow valve is stuck closed. If neither is hot, you have a thermostat stuck open and need to replace it.

there is a vacuum switch inline on the heater hose that is not opening up and letting the hot water go into the heater core.
replace the heater control valve on the heater hose coming from the water pump.

your heater hoses to your core are either bypassing the core or the valve is not working and stuck in the closed position.

You didn't list the year or engine size, but I believe your problem is the heater control valve. It is supposed to open and close to allow the hot coolant into the heater core when you switch the heat on. It will stick closed when it fails. It is located in the heater hose line, should be about $20 -$30 part.

Check your coolant level first. Next check your heater control valve. With engine at operating temp and heater on high, do this:

1. Heater hose from water pump should be hot (if not, water pump may be dead or thermostat is stuck closed)
2. Heater hose from thermostat housing should be hot (if not, thermostat may be stuck closed or water pump is dead)
3. Heater hose going to heater core should be hot ( if not, heater control valve is not getting vacuum from heater control unit, or valve is broken)
4. Heater hose returning from heater core should be hot (if not, heater core is blocked, undo both hoses to core and flush with water hose until clear)

Test thermostat: with cold engine cranked, watch water temp gauge. As it gets hotter and hotter, when the thermostat opens, the temp should drop suddenly and then go back up.

Test water pump: with engine at operating temp. Squeeze top radiator hose tight, when you release it, you should feel/hear a surge of water. It is much like when you pinch a water hose closed and release it.

Test vacuum from heater control unit to heater control valve: undue vacuum line from control valve. It should have a vacuum when pressed against you skin in addition to releasing the actuating arm on the control valve.

If there is no vacuum, a quick fix will be to disconnect the plastic actuator from the silver arm on the heater valve. This will let the hot water to always flow to your heater core.