1990 harness questions (two pics)

[FattyBoomBatty]

Sorry, I should have been more clear, the mystery plug has 4 wires, my bad.

Also, the little black one in number two, I traced both wires back and they both say, "ignition pulse detect" or something like that. What will that connect to then? I eliminated a lot of stuff and I think the brake stuff was included in the removal.

one wire from the black plug goes to the small yellow ecu connector, the other to the large white one that connects to the dash harness (in a dsm). both say ignition pulse detect. The site with some info that i've been using suggests unclearly that it could be a tach signal cleaner? What is that? Any more suggestions?