Had a K40 stealth many moons ago...around when they first released the option to lower/increase volume. Worked great at the time till lasers became more common. They're still in business and have upgraded their systems, although perhaps not as effective as some competitors.

To answer your question, i believe the beeping, if your model is as old as i'm assuming, may be normal scanning operation. There may be two dials, one to power the system that also adjusts sensitivity for hwy/city settings (larger dial) and another dial to control volume (smaller/skinny dial).

Mounting locations can be tricky, you may have to get her up on a lift and take your time. It's also possible that the rear sensor may be missing.