update: nevermind, I found it. Under the cover at the top of the firewall. Thanks anyway.
I'm embarrassed to ask this, but I've got a no start, and based on previous experience with other BMWs, the main relay is a suspect. But where is the thing? The BMW electrical diagram references it, but I can't pin down which one it is.
Edit: this a '91 E30 318ic, not an 'is, if that matters.