Removing the lower cover under the steering wheel, you can see a couple of the vac mods. Removing the glove box liner should allow you to to view other vac mods from the passenger side. I have used a piece of hose placed at my ear with the other end pointed at a suspected faulty mod. If the hissing is not pronounced, I proceed to the next. Once I locate the culprit, I then decide how much of the dash needs further dismantle. One occasion or two I have found a vacuum hose has just come undone. Good luck