Have you followed dewfpo's instructions yet and isolated the leak to either inside the engine bay or inside the cabin. That would GREATLY simplify troubleshooting.

As he mentioned all vac going into the cabin has a check valve on it so that if you mess something up in the cabin, the engine is unaffected (not neccesarily the reason they did it, but it works). My GUESS based on limitted information is that a vac line got knocked off inside the cabin whilst doing the stereo work because:

You indicated that the guy who did the breather is a 1.5 hours away away so I suspect you would have noticed the lack of the vent operation by the time you go home.

With a 1.5 hour drive, you would have tripped a check engine light if there was a vac leak.

PLEASE give as much information in the first post as possible and not dole it out in bits and pieces. First you tell us you changed out the stereo. Then you tell us someone did lots of work in an area that has a crap-load of vac lines. Finally you tell us said individual is a long drive away so we have to assume you drove it back. All these tidbits and I do not see where you have the results from dewfpo's relatively simple diagnostic test. Troubleshoot should be a step-by-step orderly process. If you jump around, you miss things.

Onkel Udo