It's on the left hand side of the console, behind and down from where the ICS is.
I've just replaced mine and the heater matrix as it was leaking and caused the ECU to fail.
Alfa were going to charge me £1400 . . . there was also one on eBay for £800+. I managed to buy an almost new one from ECUEXPRESS.co.uk for £300 (with 1 year guarantee), and it was delivered from the States within 4 days. That did the trick, and it's easy to fit . . .
PS I'm looking for a short front downpipe for my 1999 3.0. Would that chap have one ?