I need a recommendation for an indicator light (maybe mailorder?) for my
mailbox.

Here's the situation.
* It's a custom mailbox, basically a door cemented in a pillar of concrete
* It just has a square metal door with a brass inside latch (all metal)
* It's higher than my eyes in the car window by about a foot
* So I have to reach inside to see if anything is in there
* It doesn't even have one of those little red (manual) flags
* There isn't 120v lighting but landscape lighting is only a foot away

I wonder if technology can help me?

It would be nice if there was a way to mount an external LED such that an
internal light sensor activates it to indicate mail in the mailbox.

Any pointers?