Here is how I did it on my 91 E30
1. Buy the reset tool from handsontools.com
2. Remove knee bolster panel on drivers side and access the orange connector leading to srs control box.
2. Locate the white/voilet(TX) and white/yellow wires(RX). Splice in a set of wires long enough to reach 20 pin connector under hood.
3. Remove 20 pin connector and access the wires from behind the connector. Pin 15 is white/yellow(RX) and pin 20 is white violet(TX). Splice these two wires into the wires you have coming from the srs control box connector.
4. Put it all back together, plug the tool in and reset the light per the instructions included with the tool. Now you can reset it any time you want.