Don't despair! There are changes to the stock system that will improve the sound.
I simply replaced 5x7's in the doors with Kenwood separates and enjoyed an immediate improvement in clarity etc.
The input allows you to run any source you can think of.
I'm not sure whether SYNC will link with a bluetooth GPS, my GPS isn't equipped with it.
Search the forums you will find lot's of info, IMHO there are enough good features with the Shaker500/SYNC setup to make it worthwhile modifying
I agree completely, I maintained the shaker head unit/navi/sync/and blutooth, but have changed out all the speakers and added some subs and amps. The car sounds great, but still looks and functions the same as stock.
Unfortunately the OP is asking about installing an aftermarket Navi headunit, which is more difficult to integrate into the cars stock amps and speakers. It is very possible, but there are definite features of the shaker 500 (bluetooth / sync etc) that will be lost. So I tend to agree that installing an aftermarket navi headunit is pretty much an all or nothing kind of mod.
2012 GT CS Ingot Silver Vert, Edelbrock E-force supercharger with CAI and 3.5 Pulley, Tremec T-56 Mag, Accufab 84.5, Centerforce DF, Billet Yoke aluminum driveshaft, FRPP 3.73 Rear, MRT Performance H-pipe, Bassani axle backs, JL Audio sound, Edelbrock strut tower brace, JPC adjustable UCA, UMI adjustable panhard bar, KW V3 coilovers, Steeda heavy duty front mounts, Steeda rear LCA's, 20" Steeda Spyders with Michelin Pilot Super Sports, Steeda Big Brake Kit ....... it just keeps going.