BMW OEM Subwoofer Upgrade.
Late model "E" and "F" Series BMW vehicles are factory fitted with either a 6" or 8" subwoofer under each front seat. They are very shallow mount drivers made to fit into an ABS enclosure that couples with a cavity in each side chassis rail. A great concept, but they generally sound very average. German speaker manufacturer Rainbow has designed BMW specific drivers to directly replace the original units that give a massive improvement in both quality and quantity of "bottom end" response. As these images show, the Rainbow drivers fit perfectly into the factory enclosures and even have an OEM matching connector for a completely perfect installation.