On my d15 I have laying here you can't get the bracket off unless the alternator is out. :/
I know exactly which bolt you mean. I'm assuming the nut is off. Try spraying wd40 or pb blaster on the bolt and let it soak a little. Thread the nut onto the bolt slightly (3 or 4 threads) and try and tap it loose with a small hammer. Don't go crazy if you hit too hard you could damage the threads on the nut. Could also try using a screwdriver or chisel on the big square and and try prying it through
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