which vacuum line?
 

Some of you may laugh.. but I'm installing an HHO enhancer to my 99 ml320. I installed a kit on a friends car and he is getting about 40% better gas mileage. Since this is a project... and may be a first for an ML. I was wondering which would be the best location for a vacuum tap? (I'm using the vacuum as an intake rather than direct air intake). The problem is I am having trouble locating the best place for a good vacuum, close to the intake. Anyone have ideas?

(just in case the questions come up... i have a 'brick in a bottle' design, with an hho bubbler, dryer, and dual O2 Efie controller).