1984 300 TDT Fuse Blows - Eventually - Every Time

The fuse that controls the horns, ignition buzzer and backup lights on my 1984 300TDT Wagon always blows. Not immediately, but eventually. Cannot relate it to any action of mine. The backup lights were broken with loose connectors, rust and corrosion, so were replaced with new ones.

Should I check or disconnect the horns and see if that cures the problem? Any suggestion as to where to start?

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