MickAP is correct for UK car about the fog light, bigger bulb than the reverse light too.
The reversing sensors work by bouncing an ultrasonic signal off something. If the echo is less than so much it beeps, really close is continuous. If one is faulty then beep is likely to be continuous. On my kit each one has a “waterproof-connector” to plug into the black wired loom outside the car, and then the loom has plugs for the individual sensors going into the control box inside.
Here is how I installed mine and the pic show the usual type of control box and its typical connections and where Mazda expect it to go, I later moved the beeper closer to my ears.