yeah you have to ground a pin on the back of the unit. Ground pin D7 on the HVAC control and it will be in Celcius all the time. Or there maybe a wire that is run to connector C220 pin B and you can ground it there. Not sure where this connector is as I can't find the reference, but typically if the car was equipped with Daytime Running Lights, it would have this pin grounded, hope this helps.
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1994 Chevy Caprice 9C1, 126,000 miles, T56, 4.10's