Takes effort to set up, but I've recently taken to measuring volts at the ECT, not just ohms.
I found that when things are working normally, the low speed fan comes on at 0.37v coming out of the ECT, at it's connector.
Before getting to that point, tho, I like to verify these components: EEC, IRCM, fan motor and it's connector, by connecting a code tester and doing a KOEO/ER test.
As we've discussed in the past, this test will attempt to run the fan at both speeds, low then high, one right after the other. If you initiate a test while standing at the pass. side fender, near the fan, you should be able to hear it start and step up low/high very quickly. If this testing doesn't show that, then you need to stop and find out why. If all is well, then volts test at the ECT. You'll need to patch inline with test leads between the ECT and it's connector. Helps to watch the temp gauge in the dash and note the volts reading live vs. where the needle sits as the car heats up, so you have a general reference going ahead. Begin with a cold engine, if possible.
This testing assumes no codes, good thermostat, radiator, radiator cap, water pump, coolant concentration, belts, heads/gaskets, etc. etc. and just focuses on the electrical components. Use your notes to report back on readings at the ECT, but...if it is indeed the original, I'd not be shy about purchasing a new one. AutoZone sells one with a lifetime warranty, BTW