There should be a white/red wire which goes from the Temperature Gauge Sender Unit to the temperature gauge in the instrument panel. Check if the wiring is loose or corroded.
Here is a post from Chongo. It highlights the ECT sensor.
The coolant temp gauge sender is the bronze (??) looking thing below the "Co" from the word coolant. Is that what you are talking about as the sender unit. If it is then if you do it real fast, unscrew and then plug (and screw) in the new one right away, you probably can get away with not having to drain the cooling system (according to Haynes). Haynes suggest to release the pressure by removing the coolant tank cap and then reinstalling the cap back (when the engine and coolant is completely cool). Hope this helps.
http://www.contour.org/ubbthreads/showflat.php?Cat=&Board=trouble&Number=983852&Forum=trouble&Words=chongo&Match=Entire%20Phrase&Searchpage=0&Limit=25&Old=allposts&Main=983239&Search=true#Post983852 
