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The second dimmer switch is a previous owner hack job. It had to have been for some custom, special purpose that didn't stay for good. Only the previous owner would be able to tell you what it actually would have been used for.
The same would go for the brackets - unless someone can think of something common that they would have been used for.
for the weird mount beside the mirrors, it has two holes, one on each side, that means that a pin went through it for some reason. Maybe it had something that folded up or down.
for the bumper mount, it looks like a fog light mount, same as object above, with an up and down swivel.