When it comes to making music even more portable, car stereo makers are thinking small. They’re adding slots for tiny thumb drives to car stereo systems on display at this week’s Consumer Electronics Show in Las Vegas. The USB (universal serial bus) drives are about the size of a keychain and cost as little as $20 at stores such as Target and Wal-Mart. Many people pop them into their PCs and use them to store computer files. But thumb drives can store digital tunes as easily as spreadsheets–making them a great alternative to stacks of CDs.