2004 Pontiac Grand Am Gas smell from Air Vents

I started noticing a strong gas smell coming from the air vents in my Grand Am a couple months ago after I had the brake fluid changed. The smell lasted every day for about a week, but more recently I have only been noticing it after the car has sat for two days or after it rains. It seems stronger when the A/C is running, but it still smells if I just have the fan on.

I've taken it into the shop numerous times and the mechanics look at me like I'm crazy because once the car turns off the smell goes away and they can't seem to replicate it. It only smells when the car has been off for an extended period of time (8 hrs or more) or if it has rained and the car has not been covered. I did take it in once in the morning while it still smelled, and the shop still couldn't find anything. They even put in some dye and said they couldn't find any leaks.

The smell is so strong that it will literally make you sick inside the car (you can smell it outside as well while it's running). Please help!

Thanks so much.