The ABS ECU (wow lets not get carried away with the tech terms) is located in engine bay, it is part of the ABS pump assembly. Basically you have the pump motor which sits on top, an ally looking boi which the barake pipes go into and which also houses the solonoid valves under that is the ABS ECU.So bad news if your trying to find one like the engine ECU as the ABS is all one unit. Very pricey from VW but available from breakers yards for about £60