You are on the right track, in that, one that is 15 years old is probably made better than the newest ones. I have seen fridges from the 1950's and 1970's still running. Many these days will last an average of 5 years, so hold on to it.
It may not have to have 120 volts into the fan, many operate on less, even 13 volts.
My guess would be if it isn't stuck in defrost mode, the fan may be bad. Some fans will run, but run too slowly to work right.
If the machine is plugged in, compressor running, condenser fan running, are the evaporator coils frosting up?
Also keep in mind, some units will only let the evaporator fan run when the door is closed, so sometimes that door switch gets worn and won't allow the evap fan to work.

Do you have the wiring diagram?

It may be in the machine (compressor) compartment or taped to the back of the unit.

On many units the compressor, the condenser (compressor fan and the evaporator fan are wired in parallel so that when one is on they all should be on.
But some units have only the compressor and it's fan wired in parallel.
The evaporator fan actually gets power through the defrost heater and defrost thermostat when they are off (run mode) because the fan current draw is low this does not heat up the defrost heater.
You will need the wiring diagram or trace the wires back from the fan to see if this is the case.
If yes then check those two components.

CletusT may know if this is a possibility as I do not.

Well, it appears that the defrost thermostat limiter was defective. But this was not immediately apparent.

Several advisers lead me in other directions, but I finally ordered this limiter and installed it. Well, that didn't work!! Actually, I didn't THINK it worked, yet the fan was working and spinning normally.

In the end, the problem(s) were 3 at once, which is what made diagnosing the troubles so difficult.

1) The thermostat limiter was defective.
2) The thermostat limiter in 'this' model appliance HAS to get VERY VERY cold before it will allow the current to flow to the evaporator fan. This can take and did take over an hour, which I think is excessive.
3) It also turned out that the Plastic Fan Blade shaft hole had a very tiny split in it, that allowed the fan to spin on the shaft, without the motor turning and vice versa. So when the motor WAS running and spinning, the blade stayed put, making it appear the motor was not working.

So this one was not one of your normal events, and thus, the only way to figure it out, was to just keep digging. Had I known more about appliance operations, I 'might' have been able to identify the issues much sooner. But I think it would have still been a challenge. Good thing I wasn't paying someone to fix this!!! : )

One additional tip to help get the fan spinning AND bring the temperature down in the freezer is to take a small jumper wire, and jump the thermostat limiter at the by pass plug. This will allow the fan to run when the limiter has not cooled down enough to close the circuit on it's own. After about 20 minutes, I was able to remove the jumper, and the fan continued to run normally thereafter. (See illustration)

Thanks for the suggestions, and hope this helps someone in the future.