I have a '90 325i, and I've been looking for a part online. It is a piece of plastic in the wheelwell right in front of the wheel.
It seems like an internal brake duct, because it looks like it funnels the air towards the rotors.
My right side is missing one and the rubber is flapping down a bit (it looks like a reverse mud flap
If anyone has any info, or a way to get one online, please let me know.