I guess by now you've figured out it's the aft sensor that needs attention. Also, as the other responants noted, there are two different sensors available for 740's. I don't know about the differences by year, but I do know that cars equipped with EGR (like in Calif) get a different sensor than the non-EGR cars. I also know that if you get the wrong one, the car might run OK but it will throw the check engine light. This happened to my mother-in-law's 89 740 Bosch EGR car when she took it to the wrong mechanic. I'm saying all this because I'm wondering if your car had the wrong one to begin with (previous owner boo-boo??) in which case you don't want to replace it with the same as what's in there.

If you have EGR, accessing the ECT for replacement is a PITA. I'm guessing your 90 Regina is non-EGR if it's a 49 state configuration.

Reference reading:

http://www.brickboard.com/FAQ/700-900/EngineSensors.htm#EngineTemperatureSensors