pioneer dvd/navigation units - no power

[ync]

my avh-p7500 and avic-80 (nav) have been working for 2 years. suddenly there's no power to both units. the fuses are all good and there is power to them. then there's this black rectangular box between the fuse and the unit for both avh-p7500 and avic-80. the wiring is like this for both units: positive wire from battery goes to fuse, then to this black rectangular box, then to unit; negative wire from ground goes to unit and also separately to this black rectangular box. this box cannot be opened (no screw or clip) unless i break it open. does anyone know what this black rectangular box is? what is its purpose? thanks for reading.

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