There's a coiuple of places. First I'd remove the right side panel of the center console, to the right of the radio. There, you'll find a row of vacuum relays. The second one I believe is for the footwell. If you remove the vacuum line that is pointing toward the radio (it's the one going to the vacuum pod to control the footwell flap), you can put a mityvac on it and apply vacuum. The line should hold vacuum and you should hear the flap move. With the engine running and blower motor going, you should get air into the foot area. If you get nothing you can't hold vacuum, you have a leak.
Now it's time to trace the vacuum line to the diaphram. Wither it's a connection, or the diaphram is leaking. You can go here to buy a new rubber diaphram.
http://www.autoluxmbz.com/