65 Barracuda 187 code!!!

So heres a couple other things i was wondering if some people could help me figure out. This plug is on the back of the transmission and if i remember correctly on other mopars, it should have 3 prongs sticking out of the back. What does this do?

Net question is this. The electronics on the new shifter are there for a shift indicator light. You can see what its supposed to light up, and you can see the electronics. What bulb do i get and does anyone know how to wire this up right? I can live without it for the time being, and its not like the carpet is going back in for a while so i can always go back to it. My brain is a sponge lets fill it! :newb::happy1: