While inspecting my monovalve tonight for damage, I noticed a black electrical connector located in a bracket behind the valve (closer to firewall) and to its right. The part appears to have a plastic shroud that snaps into the bracket(mine is broken), with a 3 pin connection. Part number is 126-545-01-26 on the female end. I wasn't able to find much online. I checked all over the bay but couldn't find its partner.

What is this little guy? Fingers crossed it has something to do with HVAC

https://www.dropbox.com/s/1ygv4c5ooq1yjol/20180127_170009_HDR.jpg?dl=0

https://www.dropbox.com/s/12hy1d88h6e0imk/20180127_165955_HDR.jpg?dl=0

https://www.dropbox.com/s/zjhxkduajimmqsm/20180127_165948_HDR.jpg?dl=0

You have an 85 W123, right? If so, this is the electrical connector for the rack position sensor on the IP. It goes to the controller in the foot well of the front passenger. I believe part of the emissions control. When mine crumbled (the piece that had the part number on it!) I drove around with it disconnected and noticed nothing different.

Correct, '85 300D.

I can't think of anything that feels off. The IP was just rebuilt in the last few hundred miles.

Yeah, that was a single year part for the W123s. Mine crumbled completely the first time I touched it.