i bolted mines just under the screen wash tank behind the battery

and next to the cam/rocker cover
 
On my drivers side there was a threaded hole below the light unit, which I found a matching bolt for and just attached it to that. On the passenger side, there is a slot below the light unit that I slid the ballast into and then secured. That's on a mk4 though, don't know how different it will be...
 
that was on my GTI Craig, aslong as they are bolted on they will be secure mate, they have been fine since i put them in february last year.