mirror

Hi there, just a short question. It's not about TMT though, but I hope it's ok to post it in this thread anyway ;) In Blender I've started to create a roof for an old German fairy house. First I've created just one side of the house and wanted to add a mirror modifier. But something went...

This so needs to be a thing. We got spaceships, laser's, dogfighting with alien starfish, and faster than light travel...but no rear-view mirror? Wut... I'd love to have a small strip at the very top of the windshield (or a small square off to one side) that shows a view from a camera mounted...