That's the thing. The E30 doesn't have a diagnostic port because it's OBD-I. The stomp test is how you determine if their are any codes. With the ignition in position 2 stomp on the gas pedal 5 times in 5 seconds. The check engine light should flash. In a morse code style, the check light will flash indicating the code. One flash then a pause is 1. 2 flashes then a pause is 2, and so on. The 4 digit sequence is the stored code. Go to YouTube and type in E30 stomp test to see it.
As for the transmission, the filter is located inside the pan. Should be visible when you take the pan off. If you need I can show you with a diagram.