Trans Temp Gauge

Drive an 02 1500 Z71 5.3.  I want to put in a trans temp gauge.  I see that you can just replace the entire dash unit that would have the trans temp gauge built in but is there a cheaper way to do it.  I would like to do this but don't know if anything has to be reprogramed.  I found one on Ebay but that speedometer goes up to 120 and mine only 100.  Is there a way to wire up a gauge into the wiring harness for a pilar mount.  That might be easyer.
 
Also:
I look at my manual and see that the Message Center will tell you when the Trans Temp is too hot.  Anyone know what temp makes this light come on.
 
Thanks