First off, a big hello to everyone on here! (i'm new) I have a 1999 Disco II with 73000 Miles and for some unknown reason the transmission temp light is on no matter what, even if the vehicle sits for a week. Please help! Thanks.

 

Easy fix , its the temp sensor on the drivers side of the transfer box. just unplug the two wires and clean the sensor male pins with a small wire brush. put a little electrical grease on the pins and plug it back in. high resisance causes the gear-temp light to illumination
some cars have an actual transmission temp sensor located at the drivers side radiator,at the lower trans cooling line. also a two wire sensor covered with a rubber boot, same issue , corrosion on the pins

 

I unhooked the two wires and still nothing. (Light stays on) I am going to try cleaning and grease next, but if that doesnt fix it what else could it be?

 

You probably unhooked the wrong wires.

 

The transfer box sensor wires need to be connected to the sensor to turn the light off, the corrosion on the sensor pins causes the light to illuminate because of high resisistance (get it ?) unplugging it will give the same effect,light on.
Its the only sensor located on the drivers side of the t-box , and nothing is covering it or hiding it, two wires two female connectors , clean the pins on the sensor and reconnect them. same issue if the car has a sensor on the cooler line at he drivers side radiator, clean and reconnect .