Hey man.
I recently had what I think is the exact problem as you had with the gauge after pressure washing/degreasing my engine bay to diagnose an oil leak on my (nearly identical to yours) 97 LX. My gauge started reading erratically last night and soon after spiked and is stuck at the H even when the engine is off and key is out. I ordered a new harness and sending unit to my local dealer (Airport Toyota) and it should be here Wednesday.
I plan on attempting to fix the gauge the same way you did, I am in the Knoxville area as well, by any chance would you be willing to lend a hand adjusting it back? I’d be happy to pay you for your time with assisting (Money, beer, food, parts) whatever method you’d prefer. It looks simple enough but having someone who has done it before would be a huge help. If not it’s totally fine and I appreciate the very helpful write up!
I’ll shoot you a PM. In all honesty I hardly have time to work on my own these days, but I’ll gladly be a phone a friend if you hit a snag. (Though admittedly my memory is pretty fuzzy on this.).
Be sure to watch that other guys YouTube video I linked on my first post cause I remember it helping a lot in getting the cluster out at least.
my all oem parts hand assembled reproduction 82221-60010 's have been updated / improved upon a bit since last posting here ....
i use a different main round BUTTON brass sender button terminal and connector housing it self , on BOTh the main round on for sender and the SOLID one for the DEDICATED male spade terminal needed ground tab ,
this is what DOES get mixed up by a person or ship that is NOT aware , the round button connector housing can actually be easily FORCED onto the Grounding tab by mistake
the ground tab plastic connector housing is SOLID and cannot be force fed onto the round copper button on center of round bell sender to be clear here
i would have a uniquely perspective here and can accurately state the fact of certainty , that approx. exactly 1/2 or 50 % of 1FZ-FE owners find me becasue a HUMAN error is involved mix up , the old crusty harness gets blown off during a engine bay wash , or just rattles of after 20+ 30 years ....
or the tiny azz 22 gauge wire on one or both severs from age time and corrasion
the owner preforms a low tech 2 new spade female terminals crimp on , at the auto zone parking lot , and mixes them up ...
the other GOOD 50 % ish or 1/2 fall victim to shear fate and bad luck no fault of there own ,here
the copper center button connector wire severs at the brass yazaki tin terminal inside the connector housing , hence it is still attached to the untrained human eye ,
but that now frayed tinned copper fine stand wire ., conductor starts to whip around in that space cavity in the engine bay out of visual site , and it dont take long before it finds a happy litte place to get stuck in a groove or mating joint of 2 metal parts , then its all OVER from there sadly ... 
if this was intentionally done in my own T-TEN land cruiser shop laboratory under the above controlled conditions , i would and could say from KEY ON onset of the button wire finding and staying put on a neg- grounded signal ....
i would say the life expectancy of the guage pods , and blue printed circut board , and the mechanical oil guage itself , its COIL winding and bi-metalic spring that moves from thermal heas resistence pulsing signal normally when all system parts are good and correct ,,,,
is a total time window of LESS then 29 metric seconds hands down , no way those parts will take a CONSTANT non-pulsing NEG- ground full on signal ant longer then that
i have been show cell phone fumbling video clips of a real smoke cloud erupting from under the steering column into the drivers face !
i wish i had saved one , its a fascinating technical thing to see occur as is just happens in less then 29 seconds , it is best described as a TOTAL system melt down
it stops on its own after all the copper windings turn to ashes latterly , and the short circuit of B+ power has nothing left to travel through ......
that is the 411 on what does happen if and when the short circuit does occur naturally , it would be the same for a human mix up too , however , a tach at a shop usually starts a finished job up in the service bay via a arm and hand through a open drivers side window , by habit , to check there work for non related leaks and to get OIL up, though a NEW fresh oil and filter change , to recheck the dip stick and top off when necessary after the filter fills up and anti drain back valve holds that 1/2 top full qt inside depending on what filter of the week toyota decides to use ?
the time window from a in a hurry start up to backing up out a service bay to finding the first open parking hole just outside a busy shop rolling automatic FAST speed garage door motor can be a 10-11 seconds flat time window in all reality too
so you see my points above now , the nest driver , of that 1FZ-FE can be the victim of the melt down smoke cloud too !
i have been working at a Toyota service dept. back in the day when i was still with toyota and this scenerio did play out clear as yesterday a few hand fulls of times ....
not to me luckily ....
i did find broken oil sender wires before they found the ground NEG signal
i only ever had the dreaded learning on the job experience of a all day mind boggling WTF happened here that ONLY paid 1 single billable diagnoses flat rate hour ,
the junk yard delivered a used complete cluster days later , and the 82221-60010 or later 60021 was replaced too and off to SOCCAR MOMs day camp that Green pearl metallic ALL 80 in my mid Atlantic happened to be oddly >
i cant recall right now if i ever saw the kool paint codes back then , i do Remberger the kooler green upper body and pewter silver below the side body belt moldings all around or 2-toners i called them .....
you always saw this here or the below , many had a unique clear coat delamination of a original factory paint job on hood , roof and back hatch too !
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