That sensor is located on the top of the trans just beside/underneath the rear engine mount bracket, it is black and should have a gray two pole connector going into it. The harness for that sensor will be clipped onto the side of the same engine mount bracket that is near it. This is all accessible from underneath the car. It has been a long ass time since I have had to do one but I seem to recall that I used a custom ground down torx bit to get to it, take a look at it and see what it will take. For what it is worth, while I have not done one in years we changed these suckers out all of the time back in the day. It is the VSS or vehicle speed sensor, part number is 9148788 up to chasis number 266999 4 door and 170999 on a 5 door.

Mark