The E31 air intake is more complex than other models. From the top of the engine bay you can see the rubber boots on each side joining the air filter boxes to the pipework. However, there’s another boot on each side after this connecting the air ducts to the pipework. You can barely see it from above, and replacing it is probably a bumper off job!
The replacement piece was very cheap at $6 so I replaced it even though I doubt there was any real problem caused by the rip. The bumper was already off – I didn’t remove it just for this!