Have you checked the wiring from the MAF to the computer? ID the pins on the ECM connector that run to the MAF, check resistance from one end to the other on each wire. Should be very low, like 0.2 ohms. If it's very high, the wire is likely at fault, but the connection between the wire and connector's pin should be verified.
Theoretically, if you unplug the MAF and nothing changes, the computer isn't reading anything from it. Therefore you have 3 potential problems(assuming you've isolated a vacuum leak). MAF, computer, and wiring. If the MAF has 3 wires(not real familiar with BMWs yet), one should have voltage(perhaps 5, 8 or 12), one should have continuity to ground, and one should be for a return signal to the ECM. Hope this helps!