In July of 2019, I submitted a method of placing a contact sensor inside a steel mailbox. To get the triggering signal outside of the mailbox to the bridge, the antenna was extended and placed outside of the box. That system finally went down. I do not know why; it may be that the altered circuit finally gave up or the extension wire solder failed or something else.
The pictures below show how the original system was modified to a new more robust system. The circuit is altered – but in a more benign way. In the replacement system the I removed the reed switch from the sending unit and soldered in its place a pair of leads. The removed reed switch is soldered to the other end of the leads. The sending unit is placed outside of the mailbox on its bottom while the reed switch is routed inside of the mailbox to make contact with the door-mounted magnet. The installation is completely hidden from the normal outside view. If your two and a half feet tall you could see the modifications if you looked up.
If anyone is interested in the details of this new modification, let me know.