How can I find the main ecu for an astra van 1.7 td 1999?

I have a 1999 Astra van 1.7 dt and i'm trying to find the main ecu. One forum said it was at the back of the engine - not there. Another said behind the nsf inner wing shield - not there only cooling ecu. Another said in the secret compartment near the battery - not that one! It has the gm engine not the izuzu. I've had some of the dash apart, checked under the seats, baiscally everywhere in the engine bay but cannot find it. So it must be behind a shield. Can someone tell me exactly where it is please. Thanks. Chris