OK, so the point of the mag switch state thread was for this gizmo. Once I got the switches I needed, the rest was simple ... sort of.
My rural mailbox is sort of a ways from the house. Not real far, but enough that it was annoying to walk out to it if nothing was there. Not a big deal in nice weather, but in ice/snow, it can be a bore. Tried to think up different ways of seeing if there was mail from the house. Considered wireless cameras, driveway alerts, etc. Ended up going simple and so far it seems to work fine.
Mine is a large box that I rigged to have a rear door so I don't have to step out in the road. Used one Mag switch on the front/carrier side that when the door is opened it causes a DPST relay to latch energized using one of the sets of contacts to hold the relay. The other contact is used to turn an LED on that is visible from the house. When I open the back door, another Mag switch breaks the latch circuit, de-enregizes the relay and turns the LED off.
It's all powered by a small 12V SLA battery. Since neither the relay or the LED will be 'on' for any period of time in most cases, the battery charge should last quite a while.
Now, I can simply look to see the light or not.