Welcome to the forum!!!
Do you only smell fuel when the AC is on, or is it anytime through the vents???
Im curious that you said when the AC is on, because generally the air is in recirculation and not taking fresh air in......which means that you should not smell outside smells....
Either way, i would definately look for fuel in the valley as was already reccomended. Also, there is a tube that comes down on the front of the motor on the passanger side. this is the drain for the fuel filter housing. the yellow lever on the housing is what allows fuel to come out of here....if the o rings fail, fuel will leak out there....
If it could possibly be exhaust, check over the transmission. there is a pipe that leaves each exhaust manifold and goes up and meets at the turbo. check both of those pipes where the go into the manifold that attatches for the turbo. any signs of soot means the Y pipe is leaking....
Hope this helps...if not, keep posting info, we will get it figured out
