Not all transmissions are created equal, so not all transmission were built with a place to install a temperature sensor. But a temperature sensors can be installed in several pre- existing places. You can use the factory port if equipped, you can install a sensor in the drain plug hole or you can install a sensor inline with the transmission cooling lines.
Transmission Application Guide
What is the best place to install the temperature Sender in the transmission?
The purpose for installing the temperature gauge is to be able to keep an eye on the transmissions fluid temperature. So when installing the temperature sensor or transmission sender as long as you can read the fluid temperature it doesn't matter too much.
Transmission Temperature Gauge Wiring Diagram
Following the diagrams, make sure your Gauge has at least one ground wire and one positive wire. Connect positive wire to and ignition (on with key) power supply. The signal terminal needs to be connected tot he transmission temperature sensor/sender. If for some reason the gauge is reading too high, power and ground need to be reversed. If the needle on the gauge goes to the extreme right, the sender wire is grounded.