Might sound a little funky...but! Find the vac hose that goes into the firewall. That is the hose that must have vac to operate the doors that direct the air thru the heater and a/c and defroster.. Ok If you don't have a clue do this.
find the end of the hose and take it off of it's fitting SUCK on the this hose! If you get air thru it there is an open hose under the dash...so your problem is under there..
HOWEVER if the hose is tight and you cannot suck air thru it, the bad part is under the hood. The most common leak is at the cannister.. But it could be the hose leading to the cannister also. You can also start the engine and hold your finger over that hose. There should be vac from the canister to the firewall..
There are more sophisticated methods of finding vac leaks but if you don't have the equipment this will work also.
Big JIm