Empty plug identification? Pic...

[Black88GTA]

It was nice out today, so I finally got to do a bit of work on the car. Mainly cleaning (the leather looks 1000x better now) , but I was also attempting to troubleshoot my non-functional door speakers. In my poking around, I pulled back the panel in the driver's footwell, and found a bunch of loose plugs and other things.

I apologize if this has been covered (I'm sure it has) but searching for things related to "plugs" gets 11 billion threads about changing spark plugs, and I couldn't seem to find anything relevant. I know one of these is a diagnostic plug, but what about the others?

I see...

20 pin black (I think this is the diagnostic)
4 pin yellow (3 pins used) - ??
6 pin black - ??
Single wire female bullet connector - ?
2 loose spade-type terminals - ?

I think the bullet and the spade terminals are aftermarket, but I don't know what they are for. What about the other things?

-Evan

EDIT: The car is a '94 LSi