For 1990-2009 cars

When the radio ground strap is not connected, AM radio stations will be weaker and may be noticeable to a customer who listens to weak or moderate AM stations. The radio ground strap gives the audio system a longer range than without the radio ground strap. The ground strap greatly diminishes switch pops from the brake switch, turn signal, and wiper delay systems. The ground strap, used with the motor mount ground strap, also diminishes ignition noise.

If you suspect the ground strap is disconnected, visually check for connection behind the radio.