Was working on the mess under the intake on our recent 318is. The car came from CA, so I'm thinking it has to do with an emissions thing was my first thought. Nothing against CA, but I've just never seen this cable on an e30 on the east coast.
The part number:1 734 516 .3 and its stamped as a BMW part. The factory harness plug that normally goes to the AFM had this cable on it and then plugged into the AFM. I'm never convinced it's a factory part either since anyone can make a cable.

Car turned out great. I'll try to post some pics of it in another thread.