Scrum again:
http://www.mountaingoatsoftware.com/product-backlog
Basically you put all the features into a backlog on paper / text file / spreadsheet / whatever, and rank them by value to the user. You could have a great FE without rendering reflections, so supporting reflections would be a way down the list. Even making the decision of whether that should be supported, would be way down the list.
Were it my project (and I realize it's not

), I'd start by making a 'home screen' that launches external apps for every button, and does that well. As a bonus, you can kinda check off the
navigation and OBD2 features at that point since there are good 3rd-party apps for those. Then finish the phone plugin, and get that working really well. The the
music plugin. Every time you finish a task, you move on to do whatever is at the top of the backlog (which will grow over time as new feature ideas come up, and will get shuffled a bit as users learn what the issues are).
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