Developers can now sign up to create apps for GM cars via a new SDK and API. iHeart Radio has already announced its plans to get on board, and TechCrunch reports that TuneIn, Slacker, and The Weather Channel will also make GM apps. GM's program differs from Ford's Sync AppLink, also detailed today, in that apps created for it will be designed for the car's built-in entertainment system rather than linking to the driver's smartphone. It looks like the different approaches mean that in-car app experiences will be the next battleground upon which the two American carmakers continue their long rivalry.