Try to narrow down the possibilities. You say it smells in the morning when you come in & see no obvious leaks. Some wkend when you have the time try going out & putting the key in and turning it to the ON position, with the idea being to fire up the fuel pump & pressurize the line. Let it sit that way for an hour or so & then come back & check to see if *now* you have a pddle or noticable leak somewhere. It may be that there's a small leak in the line that isn't as noticable when you're driving & the airflow blows it back & away. With the line pressurized any leaks should reveal themselvs within an hour or two. After it's been sitting with line pressure I'd try to localize the odor and see if you can at least figure if it's coming from the front or rear to help narrow the search. You could have a leak in the fuel rail or any of the injectors or as JSM suggested it could very well be in the tank itself or the plumbing right there at the tank. Try to swipe a rag across the skidplate & see if you can pickup a fuel odor from it. If you feel fairly certain it's coming from the back end then yup, like JSM said then next step would be to drop the tank and do a more thourough inspection. 13yrs is plenty enough time for corrosion to have set in somewhere in the tank, connectors, plumbing, etc, etc. If you need any help or suggestions as far as dropping the tank there are plenty of guys here willing to help out.