Yes, I found it too. You need to create empty file with any feature you like and add empty /Garmin/UpdateDeviceConfig.xml
After restart you can see which features are active in /.System/Diag/html/supported_features.html
Nuvi remembers settings and files can be deleted (or nuvi delete files itself).
Reset or clearing nonvolatile memory don't remove features.

I have added only mp3 and got 3D building and 3D terrain too. But have lost volume menu, no settings for automatic volume level now.