The black wire goes with the black house supply wire, and the red wire goes with the black wire from the lighting fixture. The white wires all connect together.
Most Heath Zenith product manuals are available here: https://www.heath-zenith.com/support/owners-manuals . Searching for the SL-6273 found two versions of the wireless MP3 door chime. The direct link to the manual for the SL-6273-00 is https://www.heath-zenith.com/system/spree/documents/attachments/000/000/839/original/205371-02A.pdf?1436551434 .
Put the batteries in the door chime. To transfer an MP3 to the door chime, connect it to a computer with a USB cable. Open the folder to view files and drag the new (short, <= 20 seconds long) tune into the folder. Eject the door chime (like any USB hard drive) and remove the USB cable from the door chime.
Next press the Tune button to cycle through the tunes on the door chime for the front door chime. Stop when the desired tune is played. That will be played any time the front chime is pressed. If you add the jumper to pin 8, the door chime will ding as that is the rear rear door sound. For the front door, make sure that the switches (1-7) on the push button match those for the door chime.
I hope this helps.
Cindy Wells
Like Steve stated. I would add to use a rust-converting primer (the type that converts surface rust into an invert primer, stopping all future rusting).
As this is only occurring in the left corner, I'd try to determine exactly where the water is coming from (that's causing the damage). I suppose it could be condensation if the unit is over ten years old, but it also possible that a connection slightly loose or worn.