Marco’s 2012 Audi A7 had a silent apocalypse inside its electronics. The MMI screen stayed black, the adaptive air suspension was frozen in “loading” mode, and the key fob worked only if you held it against the driver’s door handle and prayed. The local dealer quoted $4,200 for a new Gateway module and two days of “component protection removal.” Marco, a hobbyist with a soldering iron and a grudge, said no.