Tranny swap from an 88 to an 87

[Cyberguy1992]

I'v got an 87 MKIII Turbo and I'm changing the transmission because I managed to blow the original. Anyway I have a transmission that came off a 88. When i got mine droped out i sat em next 2 eachother and mine had a sensor on the left side and kinda in the middle (and so did the new 1)I'm 99.9% sure thats for back up lights. Then on the very back mine had another sensor for IDK, but the other transmission dosn't have that sensor so I was just going to swap it over and in the process the head cracked the bolt holding that sensor on. So I took a look on the other tranny where that sensor went in @ and in had a cover over it. so im guessing the car it came off of didn't have that sensor.. I don't know if i should even mess with trying to get the broken bolt out and still transfer it over or if I shouldn't.

What is that sensor for?
Do I need it?