[Phantom]

If you go with the Coolant temp sensor What I would do is go to a junk yard cut the wires off the coolant sensor and pull the sensor.

Then make your own pig tail by smashing the junk temp sensor only keeping its connector. After the connector you would connect the cut wires to the portion where the sensor used to be. This will allow you more space to add a second set of wires or room to splice in a wire to read from (possibly just add a wire to the pin) but allow you to return to stock if needed.

So a quick run down of how it would be connected.
Stock plug> modified sensor plug - extra wires - junkyard plug> Stock temp sensor.

If you do not mind modifying the stock wiring then just open the wire coating push the new wire between the center wrap around the stock wire solder the connection and then use liquid tape to seal it. You could always try removing the pin from the connector and adding a wire at that point or shoving a thin wire in and around the pin and connecting to the sensor but those two methods you might have a hard time getting it to fit.