ECM Battery # 9 20amp fuse blows

I was driveing down the road and truck died(95 sonoma). stopped and tryed to restart. motor turned over fine would not start. checked fuses . # 9 fuse was blown. It is the ECM battery fuse. So I replaced the fuse and as soon as I turned the key on the fuse blew instantly. If anybody has had the same prob or knows how to fix I would love the help

9-1-08
I do know that fuse deals with the fuel system and that 9 out of 10 times its the fuel pump. The fuel pump is new and the wire harness inside the tank is too. I checked relay and it checks out fine. I'm at a lose here. Its been a good truck for the most part. It has 253,000 miles on it and still don't burn oil but i am to the piont where a stick of dinamite would be a good fix . lol
So if anybody has any ideas on what it could be the help would be much app.