The entire rear quarter is empty and very resonant. We fix this with come CLD on the metal surfaces, and you add material until the metal sounds more dense. Then you add sound absorption, as much as possible, and I use Ultratouch R-19 because it can be torn up and shoved in small cavities or left as a bulk piece that provides some pressure on the panels - increasing the acoustic damping properties.
A lot of work ahead, but I do enjoy this process.