Sounds like you may hve the wrong thermistor? I bought these and while they DO fit, the QC is really bad. I'm on my last heater (of 5!) in a year. 2 of the 5 almost immediately failed. Pretty sure all these heater elements are the same (probably the same source!) and so long as you find the correct thermistor resistance, you should be able to swap them directly. Per the specs on the product I bought, they are 100k ohm.

https://www.amazon.com/gp/product/B07QF1TVJ1/

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I change the thermistor every time I replace the heater element just to be on the safe side, because frankly I hate the process of replacing the parts ;)

I also have found that it is quite easy to simply solder the wires of the replacement directly to the wires on the system and use some heat shrink to seal them up when done. It is arguably the same amount of time vs. replacing the wires and redoing the wire harness, especially after doing it a few times.