Can y'all tell me what this is supposed to plug into? It comes out of the harness in roughly the same place as the 4 pin connector for the alternator, I took the top end apart over a month ago and I've forgotten what this connector does, thanks for the help.

Edit: it comes out of the harness in the exact same spot as the front o2 sensor, still close to the alternator hook up but I thought I'd add that detail just in case.

 

It's an extra connector that your car doesn't use. If you have a special emissions air pump behind your engine it would be plugged into that.

GM recycles these harnesses and not every plug will be used by all cars

 

My car is not cali emissions so I assume it does not have the air pump, the only thing back there is the ignition coil tray and the EGR valve. Thank you for your help, I've been losing my mind trying to figure it out lol.

 

Are you sure it doesn't go to the passenger side of the ignition coil tray as pictured here?
@ChibiBlackSheep

 

You have a 3400, I am pretty sure that wire is for the ICM (as you posted). GM re-uses connectors but usually makes putting a car back together like a paint by numbers (all the connectors only fit where they are supposed to fit). The Cali emissions has appears on non-Cali cars and if this was a 3800, that connector in the back might be for just that. I believe the 3400 had the cali emissions in the front (and the 3800 had it in the back).
Check you ICM and that will confirm what you found.