I know I have the US version in a box somewhere, nearly complete. I pulled it from a '91 325i. I don't have any idea of specifics, but they seem pretty standard. IIRC, this one had two relays. If I had to hazard a guess I'd say one cut the yellow/black starter wire from the key switch and the other cut the fuel pump feed wire. Likely it also used a few internal transistors to unlock the doors, beep the horn, etc... It had a few sensors, thin ribbon wiring and was installed above the glove box.
I couldn't test it as it had no remotes. Pretty sure it's the Alpine version?
If I can find it I'll snap a few pics or get that part number. From what I recall, the wiring was kinda/sorta OEM. I'm sure it was spliced in as a dealer installable option.