The largest, most expensive mobile device in most people's lives is not a smartphone or a tablet. It's their car. As microprocessors, GPS, and wireless have invaded the automobile, carmakers have begun to obsess about apps and interfaces that make such gadgetry work. Trouble is, old-line manufacturers tend to think in decades -- car platforms cost billions to develop and take years to roll out -- not trendy weekly or even daily iterations common in Bay Area software shops. To help create its new flagship computer system, General Motors turned to Boston-based consultancy InContext Design. Introduced in 2012 as the Cadillac User Experience, the system will eventually serve as the basis for so-called infotainment centers in new Cadillac, Chevrolet, Buick, and GMC vehicles, controlling everything from navigation and Internet radio to heated seats. Here's a closer look at the design process.