I installed a back up alarm (the beeping kind) as a safety measure.  They are available at most auto parts stores.  The one that I installed cost about $8.00 and splices directly into the backup wires that go to the backup light. The unit itself (about 2" x 3") square mounted in the engine compartment on the side metal framing. There are more expensive ones that supposedly are as simple as replacing the back-up bulb.
-Submitted by George Wetzel