tbm3fan
Old Man with a Hat
If you want to save money and weight, you don't need to cover all the panels. It is most effective to place in the center of each panel to dampen the main "drum-head" vibration mode (adding weight lowers the natural frequency). It also helps to block heat, so apply to firewall around engine and exhaust pipes. I think most cars today have damper material in the center of the outer door panels.
Like in this video