Schmo
P03915
Hmm. Any idea what the sound system has to do with the Nav? IIRC, the tech package and premium sound were always two separate options.
You're correct. There are cars with premium sound, but no tech package. The MCU in cars with premium sound just happens to be Nav-ready even in cars without the Tech package. Only a software install is needed to enable Nav on these cars.
Cars with standard sound, however, don't come with a Nav-ready MCU so a new MCU must be installed first.