FUEL GAUGE PROBLEM: the needle position is wrong.
REPAIR ATTEMPT: Replaced the whole instrument cluster... no improvement.

SYMPTOM: needle never goes higher than about the 1/5th-full position no matter how full the tank is... needle freezes before it reaches 1/4-tank.

The gauge jumps to life when the ign key is engaged; plus, the orange warning dot goes on and off correctly! Okay, it's getting power, so, I swapped the instrument cluster and the "new" fuel gauge does exactly the same thing. Exactly. And, warning light works correctly. Problem arose after the vehicle was stored for months. I hoped road bumps and Techron would unstick the sending unit --- no such luck.

QUESTION(s): What is the cause? What is the solution?

It sounds like you're going to have to pull out the sending unit and have a look-see.

Or if you could unplug to the sender wiring back by the tank and measure the resistance between the green/white and brown wires while someone is filling the tank, you might detect a point at which it freezes. Of course you would have to do this with a near empty tank to start with so that you can tell what happens when the fuel level rises up to that 1/5 level.

Would you happen to know if the gauge freezes at the level that the fuel was at when it was stored?

So, your guess is a stuck sending unit. I couldn't find a harness disconnect and gave up on it.

My experience was with a 744 sedan which had a harness disconnect up by the LH trunk lid hinge. I don't know what that would equate to on a wagon. If you can't find a disconnect you should be able to probe through the wire insulation with pointy probes on your ohm meter.

It's kinda sounding like the writing is on the wall though, and you will be looking forward to removing the sending unit.

https://www.brickboard.com/FAQ/700-900/FuelTankSenderReplacement.htm