According to what I read at MFi Program Enrollment: Frequently Asked Questions the answer appears to be no, you don't need to join MFi:
I want to develop an iOS app that communicates with an MFi accessory. Do I need to join the MFi Program?
No. iOS app developers do not need to join the MFi program. Everything app developers need is in the External Accessory Framework, which is provided in the iOS SDK. To access the iOS SDK, please join the iOS Developer Program: https://developer.apple.com/programs/ios/.
Now getting the specs of your car radio in order to develop a matching App would be another thing altogether.