External fuse keeps blowing

There's a 60 amp external fuse under the hood of my truck, it keeps blowing this fuse. I just bought this truck, and have not had an issue for two weeks, l bought a new stereo system, hooked up deck and drove it go a few days, checking the amp that was left in it I left my door open and drained the battery, I drove it a day and night, as I was driving to AutoZone, my stereo went dead, 30 seconds later, my Dash lights went out as well. I looked at the dummy gauge and the battery gauge was pegged to 20 volts.I pulled into parking lot, shut truck off and started check in fuses and loose wires, or damaged from my stereo install. I had no power to any thing but lights, called the guy I bought it from and he told me there was a 60 amp me external fuse under hood up against firewall. And that it blew on him once and he couldn't figure out what it was and why it blew. But he replaced the alternator think it was no good. But didn't fix the issue. My thoughts were that I did something when installing stereo. But it was cut and dry, all I did was hook it up, everything was already ran. Has I started looking at wires and checking truck out, its obvious the owner before ran brand new wiring throughout truck. All wiring it all ran nice an neat. Secured and grounded well, with fuses and fuse box seems to be new and everything has its own protection, (fusible links, relays etc.). Also note that it has a few accessories like led fog lights, toggle switch for fuel pump, toggle switch for window wiper fluid, an external horn, on dash, and a CB radio. Truck was I'm perfect working order for over two weeks and now I can't get it to run an hour before it blows that 60 amp fuse. I am wondering if the alternator has a bad exictor, or is bad/starter solenoid, or with? Battery is fairly new. 1/19. But what would drain a battery that fast and blow a big ass fuse like that? Any insight would be very much appreciated. Thanks in advance.