Most likely culprit is a wire shorting out to the body of the car and blowing the fuse. Get under the car and follow the wiring bundle from the front to the back, look for any place where it might be rubbed raw or the insulation scraped off the wire. If you can't find anything there, look where the wires come through the firewall for shorts to the firewall. If nothing there, look to see if someone has connected a stereo or some other electrical contrivance to the wiring harness (bundle), either under the dash or under the hood, the 20 amp fuse may just be overloaded from too much stuff tied into the circuit. If nothing there, try a 30 amp fuse and look carefully for any signs of overheated wiring, smoke or strong burnt odor, be very careful doing this and have a fire extinguisher handy. Last resort, pull the gas tank and unplug all the wires on the tank and see if the fuse still blows, if it does, then the problem is in the wiring, if not, then replace the in tank fuel pump, although these usually just stop working, they don't normally blow fuses. Be really careful when working with gasoline and electricity, don't make a spark at the wrong time or in the wrong place or you will have a serious situation. Best thing is to check it out as far as you feel comfortable and then take it to a professional. Good luck.
