What does an input sensor do? Anyone know and have advise?????

I have posted her before about this problem. I have a 1999 Dodge Intrepid that lost reverse. I have been hearing it might be the Input sensor. I recently had the output sensor changed because the speedo quit and it wouldn't shift, now no reverse! Does anyone know what the input sensor controls??? The car only has 84K miles!!!! Can someone give me some solid advise???
Thanks!
Bob