I had a similar issue on my '95E300D that was traced to the O-rings on the fuel lines coming into and out of the fuel filter housing and on the injector pump. Replacing the O-rings alone didn't cure the problem. Apparently, the plastic parts on the fuel lines where the O-ring sits had degraded with heat and age, and the new O-rings wouldn't seal. In my case, several of the lines were obscured by the intake plenum, thus hiding the leaks. Once the new lines were in--no leaks and no overnight fuel odor. Interestingly, my fuel smell was in the trunk after sitting overnight, eventhough no leak was found there. Also, the O-rings on the bolt that holds the fuel filter cannister to the housing get old. You get new ones with a new fuel filter, but this step gets overlooked sometimes.