Can I take a crack at this, so as to learn from my mistakes.
Are you trying to locate the back sensor [switch] in order to intiate some sort of beware signal of getting to close.
If so it should be located on your transmission, and or off of your fuse block. The signal is activated when you place the vehicle in reverse it is activated by the reverse gear.
This is typical for most transmissions, it is quite easy to locate because there should be no wires running to your transmission, besides this one.
Please go to this page and open the transmission diagram, on the right hand side.
I am sorry, I should preface this by saying the manual transmission should only have one wire.
http://www.********************************************.com/model/986catalog/986.63-b.html
Item 1.
If I am wrong please feel free to correct me, I don't mind.