96' VW Jetta - Exhaust smell in cabin

I have taken my car to a German Auto Specialist for this problem.

When the fan is on inside the vehicle taking air from the outside I get an exhaust smell in the cabin of the vehicle when I am stopped/idling, the smell is pretty strong. When the fan is circulating air inside the cabin I do not smell the exhaust. I have opened the hood and the smell seems to be coming from the back of the engine on the passenger side near the fire wall.

The shop said I needed to replace the muffler which I did…but the smell was still the same. I took it back a second time and they found two breather hoses that were cracked and replaced those as well. However that did not fix the problem. This last time they removed heat shields around exhaust system and checked for leaks, none found. They cleaned the area. They even checked fuel lines without any luck (No Leaks). They cleaned all areas around fuel lines, cleaned the frame and test drove several times. The auto center was very determined to find out what was wrong and even took it into another shop an exhaust specialist. They also could not find any leaks.

They are out of ideas. Maybe someone here has had a similar issue and can point me in the right direction.