1999 Pontiac Grand Am speed sensors

Hi, some times the tacometer doesnt work, and the transmision dont make the change, after I time the change comes but it make a impact. I think its the speed sensor, but this car have 2 speed sensors, output and input, I can locate the output sensor but I cant locate the input speed sensor, do you know where is it located. My english its not very god, sorry, and thanks for the answer.