How can you have checked for codes when you can't power the ECU, because the fuse pops? I'd suggest getting a service manual, looking up the wiring diagram and tracing it to determine the parts that are "behind" the ignition fuse. You can then go over the relevant wiring, looking for obvious signs of trouble and, if necessary, trying to disconnect all of them, connecting them back one at a time, until the fuse pops. (A less invasive approach, would be to measure the resistance across the fuse terminals, with the fuse removed, while reconnecting the parts one at a time. When the resistance falls below 15/(fuse rating), so about 1.5Ω, you've probably found the short. This approach depends on the short not being after some active part though, like a relay or something similar, which only closes when powered.)
