First the gauge, the temp sender on the engine will have 1 wire connected.
The sender is a resistor and the engine is the ground, so the needle on the gauge reads resistant to Ground on that 1 wire.
If you unplug that wire and turn on the key gauge should show below Cold because there is no Ground.
If you directly Ground that wire the gauge should show above Hot.
That will test if the gauge is working, not the sender just the gauge.
Next, the thermostat, 195degF model is what needs to be in there, if you don't specify the temp you will often get a 180degF.
This means the radiator will not be used until upper engine temperature has reached 195degF, the thermostats job is to block flow to radiator.
To check if thermostat is working correctly, start cold engine and feel upper rad hose, it will be cold, after engine idles for a few minutes feel it again, it should still be cold, if it is warming up with the engine then thermostat is open which is wrong.
Upper rad hose should not get warm until thermostat opens at 195degF, above 1/3 on gauge.
A working Ford temp sender and gauge will be about 220degF when needle is in the middle.
So after first startup the needle should steadily climb to above 1/3 and below 1/2 on the gauge, this should take no more than 5 to 8 minutes depending on outside temp.
Feel the upper rad hose, that's the best way to tell if t-stat is working.
Also check the engine fan, when engine is cold and off, fan should spin easily.
The fan clutch unit will fail in the lock position, this is the "fail safe" mode, to prevent overheating, but will also cause over cooling if it happens.
The fan clutch is heated up by the radiator temp, not engine temp, so it should spin easily until rad is warmed up by engine running above 200degF for awhile.
No, you are correct, clogged heater core wouldn't be your problem, a partially blocked heater core can cause random temp gauge fluctuations but these would make needle go above 1/2 then back down to just below 1/2, not down to "cold".
The Lima 4cyl engines are not great heat generators at idle, and do tend to run on the cold side especially when outside temps are cold, you can add "water pipe insulation" on some of the longer heater hoses, this holds in the heat generated by the engine.
On the '95 and up Rangers I believe the dash has to be partially removed to replace the heater core, I think shop manual says 5-6 hour job, on my '94 it was a 20min. job.
So in your case if the core is not leaking yet, I would try Back Flushing the core to make it last as long as possible.
Google: back flushing heater core
Next time engine is warmed up open the hood and feel each of the two heater hoses at the firewall, one will be warmer than the other, the warmer one is the IN hose, the cooler one is the OUT hose, if you are not sure, start engine and turn heat to HOT and Fan on High.
Now feel the hoses again.
Mark the IN hose and mark the firewall IN hose connection.
Cold engine, open rad cap to relieve any pressure
Remove the two heater core hoses at the firewall and plug the hoses or raise them up to prevent coolant loss.
Use garden hose with low flow and put it on the OUT hose connection on firewall
This will push out any larger debris the way it came in, the back flush part
You can also mix up some CLR(or vinegar) in warm water and pour it into the core, let it sit for 20 min. the back flush core again, repeat as you see fit.
Increase garden hose flow to make sure flow in and out of the core is the same, so no blockages.
Refill core with coolant or distilled water before reconnecting hoses, this will prevent an air lock in the system.
Welcome Visitor! Please take a few seconds and 
