After i posted this i decided to use that cute little blinking red button in the upper right hand corner....here's what my SEARCH yielded
the plastic latch is known to break on the S13...you'll have to get a replacement cable with fuel door latch.
to remove and replace the cable, you'll have to disconnect the cable for the trunk latch.
then remove the rear seat and release the cable from the handle next to the seat.
the passenger side interior panels will also need to be removed so that you can replace the cable.
feed the replacement cable through the interior and play with it til you can pull it through from the rear side of the car.
connect the end of the replacement cable to the trunk first, then do the fuel door.
don't tighten the plastic nut on the fuel door too tight or you'll break it and end up with another broken fuel door latch. just enough to have the latch hold is fine.
then connect the line to the latch next to the seat and put everything back together.
nissan dealer quoted $200 for this job...i bought a replacement cable from a forum member for $30 and spent about 1-2 hrs.