I first replaced the rubber seal around the nozzel of the tank and put in a new fuel cap. And no more leaks. I've concluded that the diesel was escaping through the fuel cap, the diesel then passed through the old rubber seal (Which was old and cracked) and followed the nozzel of the tank into the trunk, and that's why there was diesel in my tank.
Total Cost to Fix: $6 (Fuel Cap & entrance to wrecking yard for rubber seal)
Thanks for the comments, you have saved me plenty of diesel.