What is this black box to left of fuse panel, 85 Caprice ?

It is a 3 x 3 x 1 rectangular box which plugs into a connector just to the left of the fuse panel, at the driver's left foot. Whenever my horn stops
working, I have to jiggle this black box, or the smaller white box next to it, to reactivate the horn.
Must be a loose connection in there. How can I tighten it up without ruining it? Do I need to buy a new black box ?
The car is a 1985 Chevy Caprice
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