There is also a couple of other places i would check that come to mind..... There is a ground disconnect point behind a plastic cover in front of the battery on the carpeted section that the rear seat sits on. Take that cover off and check the ground connection there. I think your e34 also has a connecting point under the dead pedal where they originally had a battery disconnect switch when the car was being transported from the factory, the switches are removed at the dealer but the connecting point is still there and if it worked loose or got corrosion on it it could have an effect also.