Why does car keep blowing 40 amp fuse?

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Car is slow in acceleration; windows won't roll up/down; AC won't come on-the fan blows but only out the top & bottom vents. The car has 208K miles on it. It has been running well. Spent $900 to have extensive electrical tests done, but it didn't fix the problem. My wife drove off the lot and problem returned. I won't take it back to mechanic until I get specific info of what problem might be.