2000 Buick Lesabre electrical or ignition

If you went right to the wire that comes from the battery and bolts to the fuse box, there is no fuse there, but you'll want to check that wire to see if it melted apart. You can identify that by tugging on it. More likely what happened is you created a bad connection on that point. That has been a big problem on almost all car brands since they started doing that. Pull your added wire off, then be sure that connection is clean and tight.

I have to tell you that this is a very, very bad idea. The high current draw of the fan will cause all kinds of electrical issues. I'm betting that your fan circuit never had anything wrong with it in the first place. These fans aren't even programmed to come on until approx 225 degrees or higher and that is the way they are supposed to operate. Everyone panics when they see the temp rising but that is entirely normal. As long as the fan comes on before hitting the red zone and it cycles back off on it's own, it is fine. If the fan comes on with the AC, it is probably OK. Either way it should be repaired properly, not hot wired.

Check the fusible links at the starter.