[papajam]

The only caps I'm aware of, other than the one in the dizzy, were mounted on the alternator and/or ignition coil to suppress RFI. And even then they were not installed in all years/models. It could be, since it's by the small fusebox, that it was installed to suppress fuel pump RFI. But that would be the first I've heard of it. Perhaps it was installed by a previous owner for some sort of sound system or something. Does it look like a factory install?
In any case, if everything electric in the car works, and since the wires been cut, I'd toss it in the bin.