Two ECT Sensors LS1 Where is the other one?

[Corveric]

We have transplanted an LS1 from a '98 Silverado into a '97 (originally was a 4.3) and we used a computer and harness from an '00 Vette LS1. (Or so i am told)
The harness has two engine coolant temperature sensors, a multi wire that goes to the driver side head, and another single wire that is somewhere around the back of the engine on the driver side. With the engine in, there is no room to get one's head down there to look.
Can anybody help us find the sensor that ought to be there?
There is also another sensor which hooks up to the electric fans which is all looked after. I believe that the one we are looking for will operate the dash board gauge.