Where is the automatic transmission ECU located?

Hello,

The Transmission Control Module, TCM, is located under the center console, heater box, on top of the center tunnel. See image 1.

The TCM has a single, 45 pin black/brown, connector.

Pin out at the connector for 12V supply and ground.
Pin 16, red wire 12V from fuse F2.39 on instrument panel fuse block, see image 3.
Pin 31+32, brown wire ground.

You can check these using a voltmeter.

How to use a voltmeter:
https://www.2carpros.com/articles/how-to-use-a-voltmeter

A good link to location and part numbers:
https://nemigaparts.com/cat_spares/epc/vauxhall/t98/p3/1/#google_vignette

I hope this helps.

Cheers, Boris