A buddy of mine has a 1994 pick up, on which the temperature gauge is non functional. What is the most likely culprit, the gauge itself of the sensor? Also, where is the temperature sensor on these engines located?

Depends on the location. On the V6, the sensor located at the very back of the engine. Between the engine and the firewall. Put a rug or something on top of the engine and lay on it do you can get a good look at it. It's not easy to get at.

I believe it's an I4.

In that case. Don't know. Start digging around under the hood and look for it. And/or go to your Toyota dealer and order a new sensor and have them print out the fich showing it's location.