[dljags55Olds88HT]

The Car spent all of its life in Texas until now so this feature was probably rarely used. There is no heat entering the passenger compartment, defrost pulls all the way out, but only blows cold air.
The far right pull knob does not come out very far, but I'm having trouble trying to figure out what/where it is actually pulling at the other end to help it along.

I see no Fluid Control Valve under the hood, just hoses going into the firewall.
Drawing in the Manual is hard to distinguish what should be going on, just basic routing of the cables. Description of the system operation is vague, at best.

Help me get some heat so we can enjoy the fall in Michigan before we HAVE to put it away for the winter.

[dljags55Olds88HT]

Ranco Valve is located slightly right of center under the dash behind the glovebox cardboard.
Visualize it as you reach from underneath the glovebox:
____________
|========X=|
|==========|
--------------------
You can reach from underneath to feel the cable movement, it should have a slight "notchiness" to maintain position as the cable moves it through its rotation. The outer portion of the cable has to be secured properly for the internal cable to react against to get full motion.
Confirming that the cable is moving the valve through its motion is only one step.

This car never been near the threat of freezing had the cooling system filled mostly with plain water, a bad choice of economizing. Coolant protects against corrosion and lubricates the water pump bearings, but you all probably know that already.

Flow through the valve and the heater core may not be occurring due to corrosion blockage built up while being closed for so long. Another thing to suspect is that there may have been a heater core leak at one time and the circuit was intentionally plugged in one of the hoses, rather than repair the core. Some hose removal and investigation along with a full backflush should determine whether the core itself may have an issue. Supposedly, if the valve, itself, were to be faulty, they can be rebuilt or are somewhat readily available.

Full credit for this info goes to Senior Member: Bhigdog
I'm just posting the reply to add to the Knowledgebase for the next person.