As I understand it, the sensor on the front of the engine (by the firewall) is for the radiator fan (so it's actually a switch), the one on top of the head is for the gauge/idiot light, and the one in the thermostat housing is for the ECM. I'll stea... er, "borrow", a few images I found at PFF:
This is what the sensor I bought looks like (the one marked "computer"):
But my actual sensor looks like the one shown here (circled in red):
I'm just assuming that the 84 used a sensor plug that's no longer made, and the newer style is used on later Fieros. The sensor is probably the same, just the plug is different. I was just wondering if anyone could confirm this.