This is more of an electrical problem rather than a plumbing issue. But rust inside the junction box is a sign of water getting into it. You need to find the source of the water that's getting in and stop that too. It looks like there is only a bad connection inside the wire nut. Most likely due to being wet. You can try changing the wire nuts if you feel comfortable with electricity.
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Switch the breaker to "off" and lock it out & tag it so nobody turns it on accidently.
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Verify power is off to both wires with a meter or voltage indicator.
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Cut back the wires until you have a good section of wire to strip and install new wire nuts. (Remove one wire nut at a time so you don't mix wires.)
If you are not comfortable with electricity please do not attempt this repair and contact someone who is.
Great work. Now just keep an eye on it to see what was causing the moisture or water to get inside the electrical connection to cause it to rust inside. May be a leaking connection or valve. Happy new year.