By starter immobilisation relay, do you mean the one that (hopefully) prevents thieves from starting the car? I think this relay is behind the left hand kick panel (i.e. passenger side for UK, driver side for US and Europe). This relay is only fitted to later model cars (I think sometime after September 1993).
My car is too early to have the anti-theft, but it does have the starter inhibit relay (to prevent starting unless the auto geabox is in Park or Neutral) in the front power distribution box. Let me know if you want more details on this relay and I'll count it's position (or might be able to take a photo).