Electrical problem

Ok. So I own a '92 Taurus. My intermittent wipers quite working and before I could figure out what was wrong the turn signals quite. I replaced the switch which controls the lights and wipers (Its all one thing) and the turn signals still down work. I replaced the Turn signal and four way flasher control box. The turn signals still don't work, but four ways do, they have been working even when the turn signals stopped. I replaced the brake lights with 3357 lights because some of them were burned out. Then I discovered that the rear defroster didn't work. I replaced the fuse. The turn signals worked and so did the defroster. It was a rainy day, so I had to use the windshield wipers and the headlights, and every single time I turned the low beams or high beams (didn't matter) on the defroster fuse would blow. So we checked the amperage and the voltage and it reads like it's supposed to 14.8 to 15.01. So we tried putting a 20 volt fuse in. It blew. Now every time I turn on the car and I put a fuse in for the defroster it blows. We even disconnected the defroster, it blew again, and the headlights, it blew again. And now the rear backup lights don't work and I have another brake light out. Whats going on!