Wow, Vacume leak????, does it do it with the venting turned off, i.e everthing turned off, fan to off, vents set to cold, and climate set to off.??
Is there an aftermarket stereo in it???.. If so the climate control cluster is right above, maybe when they installed it maybe one of those small vacume lines got pinched or cut???.
If not, it'* easy to check!. Your fake wood paneling that runs around the radio and over the steering wheel to the left side will pop off with a little force ( there are just metal clips holding into place.) Then there are a couple of screws that hold that insturment cluster in the dash, and while your at it, unscrew and pull out the radio for a little eye room and see if there is anything not hooked up to the climate control, or like I said pinched hoses? cut hoses or maybe even not hooked up?.