what would be a good and easy unit to use/install oneself as a backup sensor, to
determine distance from wall or other obstacle (ie. pillar in public parking garage or
other car)?

I'm not afraid of doing a little drilling myself but in visiting the local napa
autoparts (also advanced auto parts), they don't really know anything about the
products to tell the customer, just point to the box on the wall

I don't need a camera, just something that would tell me how far the rear bumper is
from the wall