Honestly, I don't really care to see pathfinding implemented right on the car player. Although it would be neat to see implemented, I wouldn't tend to use it very much.

I'd rather use someone else's carefully engineered (and commercial-quality) mega-reliable pathfinding engine, and download *that* path to the player.

In fact, I think the MapsOnUs solution is the best possible "basic architecture" for my needs. It does exactly what I want it to do... If only MapsOnUs didn't suck so bad...

I like the idea of intercepting the Garmin download and converting it into GPSapp file-format. That sounds like it would be about the same (architecturally) as the MapsOnUs solution, but with better mapping data and better pathfinding.